Thursday, May 28, 2020

Avoiding noise issues when operating power supplies in parallel


Operating power supplies in parallel is commonly used to increase the available output power or to provide system redundancy in the event of a power supply failure.  Not all power supplies can be connected in this manner, so please consult the manufacturer’s documentation first.

One method is “active” current sharing and is available as standard, or as an option, on most power supplies rated 1,000W or higher.  An internal high impedance circuit monitors the output current and compares its voltage level to the other units via a parallel connection (PC) wire.  A common connection (COM) serves as the return wire.  The power supply output voltage is automatically adjusted up or down to balance the output currents between the paralleled power supplies.  See Figure 1.


Figure 1: Two power supplies connected in parallel

At first glance, this looks perfectly acceptable. When the system is operating however, it may be observed that the power supplies are oscillating very slightly.  This is not a function of the power supplies malfunctioning when in parallel, but actually may be due to noise pick-up or noise currents circulating in the wiring.

The first thing to check is that the wiring between the PC and COM connections is twisted, short as possible and routed away from any potentially noisy cables.

If that does not solve the problem it may be due to how the negative remote sense is wired.  Looking at Figure 2, we can see that the COM connection is internally connected to the negative sense connection and as a result creating a potential noise loop.  The oscillation may be the result of the currents flowing in the negative load cables affecting the sensitive parallel and sensing circuitry.


Figure 2: Noise loop within the parallel, sensing and load system wiring

The solution for this is to disconnect the wire between the two COM pins and connect the negative remote sense wiring at the load. It will also compensate for voltage drops in the negative load cables.  See Figure 3.


Figure 3: Negative remote sense cables connected to the load and COM link removed

With this configuration, the internal signal common is isolated from noise on the power cables and any voltage differentials. If desired, the positive remote sense terminals can be connected to the positive of the load to compensate for voltage drops in those cables too.

Power Guy


Wednesday, April 15, 2020

How can I increase a power supply’s hold-up time?


What is hold-up time?

When an AC-DC power supply’s input voltage is interrupted, during a brown out condition or a very brief power failure, the DC output will only remain within regulation for a short period of time.  This is specified on the power supply datasheet as the hold-up time.  During this hold-up time the power supply relies on energy stored in its capacitors to maintain operation.

Figure 1 shows a simple block diagram of a power supply.




Figure 1: Power supply block diagram

The AC input voltage is filtered, rectified and boosted to provide a DC bus voltage of around 390V. The DC-DC converter section of the power supply provides primary-secondary isolation and reduces the 390V bus down to the desired DC output voltage.  The function of the high voltage bus electrolytic capacitor C1 is to reduce the ripple voltage on the 390V bus and store energy to keep the DC-DC converter operational during brief interruptions to the AC input.

Depending on the application, the length of the hold-up time varies from a half or full cycle of the incoming AC 50/60Hz voltage.  This is typically 8 to 20ms at 100% load, but is normally sufficient to avoid electronic equipment from having to restart or reboot.

When is extended hold-up time required?

The medical industry’s concern regarding hold-up time has increased since the release of the EN 60601-1-2; 2015 (Ed4) immunity standard.  Primarily created to address the growing number of products used in home healthcare, this standard specifies multiple AC voltage dips ranging from 20ms to 5 seconds.  The longer outages are addressed by batteries or ensuring that no harm will occur to the patient or operator if the power supply output voltage drops out of the regulation band.

Airborne equipment is covered by the DO-160 standard.  Section 16 refers to power input, simulating conditions of aircraft power from before engine start (using auxiliary ground based power) to after landing, including emergencies.  The requirement is for a hold-up time of at least 200ms.

What techniques are available to increase hold-up time?

There are various methods to extend power supply hold-up time, each with advantages and disadvantages.  As the amount of energy stored in a capacitor C is calculated as: ½ x C x V2, to increase that energy storage and hence the hold-up time, either the amount of capacitance or the voltage on the capacitor has to increase. Since V is squared, an increase in the value of V will have a greater impact.

The following examples are based on a hypothetical system requiring a load of 150W at 12V, with a minimum output voltage requirement of 11.5V as the output decays to zero; 200ms is the desired hold-up time.

1.      Using a higher rated power supply and operating it at a reduced load

If we were to use the 150W 12V output TDK-Lambda RWS150B-12 power supply and operate it at 100% load, we would have a hold-up of just over 30ms based on the evaluation test data.  See Figure 2.


Figure 2: Hold-up time versus output load for the RWS150B-12 power supply

As expected, the hold-up time is dependent on the output load drawn. The greater the load, the quicker C1’s stored energy is depleted. To obtain 200ms hold-up, we could use the TDK-Lambda’s 1500W rated CUS1500M-12 power supply. See Figure 3.


Figure 3: Hold-up time versus output load for the CUS1500M-12 power supply

The larger value of “C1” in the CUS1500M-12 would provide enough energy to hold-up a 150W load for over 200ms.

This would be a simple off the shelf solution, but a much larger, more expensive power supply would be required.

2.      Adding capacitance across the power supply output terminals or load



Figure 3: Adding load capacitance

At first glance adding additional capacitance (C3) across the output seems like an easy solution.   Based on the equation 2 in the appendix, however, C3 would have to be a massive 4,595,745uF.

C = 2 x Pout x t / (V2 – Vend2)

C = 2 x 150 x 0.18 / (122 - 11.52)

Note as the power supply already has 20ms of hold-up capability, t is reduced to 180ms (0.18s).

Even with a supercap this would consume a large amount of space.  One other major concern is the over-current characteristics of the power supply during initial turn on.  An uncharged C3 would appear to the power supply control circuit as a dead short across the output.  The power supply would most likely fail to establish a 12V output when initially turned on.

3.      A customized power supply with a larger high voltage bus capacitor (C1)

As previously mentioned, the energy stored in a capacitor is equal to ½ x C x V2, so adding capacitor C1a across the high 390Vdc bus has a greater effect than adding capacitance across the 12V output.  See Figure 4.



Figure 4: Adding high voltage bus capacitance

As the DC bus is not accessible on most power supplies, adding capacitors would involve creating a custom or modified standard design. This would involve engineering charges and safety re-certification fees, plus time to make the modification.

If the power supply to be modified had a hold-up time of 20ms, increasing it to 200ms would require adding the equivalent of nine more C1 capacitors.  As C1 typically occupies 5 to 6% of the internal space of a power supply, it would increase the size of the power supply by around 50% for the same product height.

A power module based solution could also be created using a non-isolated AC-DC 360Vdc output converter (TDK-Lambda’s PF series) and a high voltage input isolated DC-DC 12V output converter (TDK-Lambda PH-A series). This again would require a custom board design with engineering and safety certification charges.

4.      A 48V or 60V output AC-DC power supply and a wide range input DC-DC Converter

For an off-the-shelf standard product solution, a 48V output AC-DC power supply could be used with an isolated wide-range input 18 to 75V DC-DC converter with a 12V output.  See Figure 5.  The DC-DC converter may require a heatsink or cold plate for cooling.


Figure 5: 48V or 60V power supply and an isolated wide range input DC-DC converter

In addition to the hold-up inside the 48V output AC-DC power supply, capacitor C1 can be used for additional energy storage.  We have taken advantage of the energy storage formula ½ x C x V2 as V is now 48V rather than 12V in scenario 1.  The efficiency of the DC-DC converter is considered as 90%.  The hold-up of the AC-DC power supply is 20ms, requiring an additional 180ms (t).

C = 2 x P x t / (Eff x (V2 – Vend2))

For our example, the required value of C1 would be 2 x 150 x 0.18 / (0.9 x (482 – 182)) which calculates as 30,300uF.     

If the 48V AC-DC power supply was replaced by one that had a 60V output, the additional capacitance could be reduced to 18,315uF. A 72V output power supply would reduce that further to 12,345uF.

5.      A 48V output AC-DC power supply and a wide range input non-isolated DC-DC Converter

In this case, the chosen DC-DC converter is non-isolated (for higher efficiency, smaller size and lower cost) having a wide input range of 9 to 53V. See Figure 6. A non-isolated DC-DC converter would not require heatsinking.




Figure 6: 48V power supply and a non-isolated DC-DC converter

The efficiency of the non-isolated DC-DC (TDK-Lambda i6A series) converter is considered as 96%.  The hold-up of the AC-DC power supply as 20ms, requiring an additional 180ms (t).

C = 2 x P x t / (Eff x (V2 – Vend2))

For our example, the required value of C1 would be 2 x 150 x 0.18 / (0.96 x (482 – 92)) which calculates as 25,304uF.

Summary
Method
Additional capacitance
Pros
Cons
Larger power supply
None
Standard part
Cost and size of bigger supply
Larger output cap
4,595,745uF
Standard part
Size and potential start-up issues
Larger HV bus cap
9 x C1 (~9,000uF)
Custom solution
Eng. cost and development time
48V AC-DC + isolated DC-DC
30,300uF
Standard parts
Space for DC-DC and heatsinking
60V AC-DC + isolated DC-DC
18,315uF
Uncommon 60V AC-DC
Standard DC-DC
Space for DC-DC and heatsinking
72V AC-DC + isolated DC-DC
12,345uF
Uncommon 72V AC-DC
Standard DC-DC
Space for DC-DC and heatsinking
48V AC-DC + non-isolated DC-DC
25,345uF
Standard parts
Space for DC-DC

Appendix - Calculating the required capacitance

Below are the equations for calculating the required hold-up capacitance.  The units of measure are Energy – Joules, Power – Watts, time – seconds, Voltage – Volts, Capacitance – Farads, Efficiency – Percentage.

Energy E is the product of the output power Pout and time t

Equation 1:
E = Pout x t

Stored energy (E) is half the product of the Capacitance (C) and the voltage (V) squared

E = 0.5 x C x V2

Rearranging for C we get
 C = 2 x E/V2

Note: The voltage on capacitor C will drop as it discharges into the load.  At some point the voltage will be too low for the load to function leaving unused energy in the capacitor.  That voltage point will be referred to as “Vend”.  In the scenarios 2, 4, 5, 6 a voltage of 11.5V is used.

Our “useful” energy for hold-up will be the initial minus the remaining energy.

E = (0.5 x C x V2) – (0.5 x C x Vend2)

Factoring the equation we get:
E = 0.5 x C x (V2 – Vend2) or C = 2 x E / (V2 – Vend2)

Substituting for E from equation 1 we have
Equation 2:
C = 2 x Pout x t / (V2 – Vend2)

In cases where a separate DC-DC converter is used, that converter will not be 100% efficient (Eff) and we need to adjust the value of our AC-DC output power Pout to compensate:

P = Pout / Eff

Equation 3:
C = 2 x P x t / (Eff x (V2 – Vend2))

Thursday, February 20, 2020

How do I evaluate a convection cooled power supply’s performance?

I discussed convection cooling in a previous article, giving guidance on how to position (mount) a power supply in a system.   It did not cover how to evaluate a product’s performance though, a subject worthy of mention as testing methods across the industry differ significantly.
First, a quick recap on convection cooling as it is widely assumed a convection cooled power supply does not need any airflow to operate.  One definition of convection is “The transfer of heat by the circulation or movement of the heated parts of a liquid or gas”.  In our case – the circulation or movement of hot air.

An open frame power supply is typically mounted on a flat surface upon standoffs.  Figure 1 shows how the air behaves.  As the hot air rises, cooler air is drawn in from the sides.  Although the airspeed is quite low, just 0.3m/s, it is sufficient to reduce internal temperatures.



Figure 1: Natural airflow around a convection cooled power supply

Before making the choice of vendor and power supply it is advisable to download not only the datasheet, but also the manufacturer’s evaluation report, reliability data, application information and safety files for the “Conditions of Acceptability”.  If it is not available from the website request it.  This “homework” is paramount to ensuring you are not going to have program delays due to non-performing power supplies or worse, excessive field failures.

The report listing the thermal measurements is a key section and may raise some suspicions.

Most power supplies are capable of operating over a wide range input.  From a thermal aspect 115Vac is tougher than 230Vac, due to the higher currents (I2 x R=losses) in the input filtering and rectification circuitry.  If the report only states results measured at 230V, this may be an issue.

Convection cooled power supplies often have a higher forced air cooling rating stated on the datasheet.  In this case the report should confirm if the test results were recorded with convection cooling.  If that statement has not been made it should be noted for your own follow up testing.  Check that the output loading is at 100%.

The mounting orientation affects the thermal performance of the power supply.  This too should be identified in the report.

Where was the ambient temperature measured?  Some reports show the temperature recorded above the power supply.  This falsely overstates the true ambient temperature. Ambient should be recorded on both sides of the power supply, or underneath for a vertically mounted or DIN rail mounted supply.

At what ambient temperature were the measurements recorded at and how long was the power supply running before the tests took place?

A convection cooled power supply will take between two and three hours for the temperatures to stabilize.  The larger, hotter components like the isolation transformer and power semiconductor heatsinks have a greater thermal mass than a small capacitor.  If the report states that the power supply was run for one hour at 25oC and for one hour at 50oC then there should be at approximately a 20 oC delta between the two sets of results.  Power semiconductors do operate more efficiently at higher case temperatures, and the natural convection airflow will be higher, so there will be some discrepancies.  This is why extrapolating results can give errors due to non-linearity.

Having received your sample power supplies, you can start testing.  If a thermal chamber is not readily available, it is advisable to bench test with the unit covered with a large enclosure.  This avoids external airflow from air-conditioning systems affecting the results.  Even in a thermal chamber, there may be an internal fan to circulate air.  TDK-Lambda recommends placing the power supply in a sealed enclosure to shield that air from interfering with the results.  Remember to test at all the input voltages your system will be operating from.

Ideally the temperature of each electrolytic capacitor should be measured using thermocouples attached to the exposed metal case at the top of the component.  This data can be used to determine each capacitor’s life.  TDK-Lambda can assist, supplying a sample with thermocouples already attached*.

In the end system it is very important to ensure that there is adequate space for the air to be drawn in from the sides and allowed to exit above the power supply.  A distance of 50mm is considered safe, less than this will cause interference with the natural convection airflow.  Power supply thermal testing has to be repeated again in the end system to verify the naturally circulating airflow has not been restricted.

The cooler a power supply operates, the longer it will last.  Care taken during the early stages of product development can avoid last minute launch delays.

*A follow-up article will be available shortly on assessing capacitor life.

Power Guy

Friday, December 6, 2019

Can I operate my three phase power supply from WYE and Delta AC inputs?

Before addressing how we connect, or even if we can connect, a three phase power supply to an AC voltage source, I think we should review some background information.

AC-DC power supplies that are rated higher than 2.5kW frequently have a three phase AC input.  In the US the voltage can be 208/220Vac or 480Vac.  In Europe it is a “harmonized 400Vac” which in actuality is 415Vac in the UK and 380Vac for Europe.  A higher input voltage allows more power to be drawn from the incoming AC at a lower current.  These three phase AC voltages can be one of two configurations – Delta or Wye (pronounced “why”).

The following should also clarify what three phase input voltage would be best suited for a large power system.  Just as important, how to read a manufacturer’s datasheet to make sure that power supply can be used in the US, Europe and globally.


Typically very high voltage power is transmitted from the power generation plants to local substation transformers (where it is reduced in voltage) and then to facilities in a Delta configuration (Figure 1).  Note that a Delta configuration only uses three wires and has no neutral or ground wire.  This saves the cost of a fourth wire, which is not needed during transmission.


Figure 1: Delta wiring configuration (US voltage shown)

I will start with the US first.  Figure 2 shows a basic overview of what a manufacturing facility receives from the Grid, at what point it is lowered in voltage and how it is distributed to the loads.



Figure 2: Typical US facility power distribution

Starting at the left, a 480Vac Delta three-wire feed enters the facility from the substation.  From the incoming distribution panel, 480Vac Delta is supplied to electrical equipment needing a large amount of power.  Large ovens, test equipment for semiconductors, burn in chambers and machines fabricating metal (including laser cutting and additive manufacturing) would typically use 480Vac Delta.  It is important to note that equipment connected to this voltage feed can reduce the size of the Delta-Wye step down transformer, saving cost, energy losses and floor space.

To supply the rest of the facility, the three phase is reduced from a 480Vac Delta configuration to a 4 wire 208Vac phase to phase Wye configuration (Figure 3) via a step-down Delta-Wye transformer.  


Figure 3: 208Vac phase to phase Wye configuration

From the distribution panel, in addition to being able to supply 208Vac phase to phase, 120Vac is available by connecting to either one of the Lines (L1, L2 or L3) and neutral N.

As very rough order of magnitude, 208Vac three phase would be used for mid-sized loads, greater than 5kW and less than 25kW and single phase 208Vac for smaller loads greater than 1.5kW.  We are all aware of the 120Vac wall outlet which can support around 1kW.  The amount of power depends on the wiring size and fusing, consult your local qualified electrician!

There may also be a second Delta-Wye transformer in some facilities.  As discussed in another blog, this provides 277Vac feeds to lighting and HVAC (Heating, Ventilation and Air Conditioning) equipment.

In Europe the arrangement and voltages are different than the US, see Figure 4.


Figure 4: Typical European facility

Again starting on the left, high voltage (11kVac in a Delta configuration in the UK) is provided by from the Grid and a step down transformer delivers three phase in a four wire Wye configuration to the facility’s distribution panel.  See Figure 5.  As explained earlier 380V/220Vac is mainly used in mainland Europe, 415/240Vac in the UK.



Figure 5: 380/415Vac phase to phase Wye configuration

From the distribution panel, in addition to being able to supply 380/415Vac phase to phase, 220/240Vac is available by connecting to either one of the Lines (L1, L2 or L3) and neutral N. 
Returning to the subject of three phase AC-DC power supplies, we shall review some examples from TDK-Lambda’s product offering.

The HWS1800T-24 is a 1.8kW rated power supply accepting a 170-265Vac 3 phase input.  This would be suitable for operation on a standard US type of 208Vac three phase Wye input.  It could also be operated in Europe, but would require a 400Vac to 220Vac three phase Wye-Wye step down transformer.

The TPS4000-24 is a 4kW rated power supply accepting a 350-528Vac 3 phase input, either Delta or Wye.  This would be suitable for operation in the US and in Europe without the need to change connections to the power supply, or additional transformers.

The Genesys+ series of programmable power supplies has a large number of models ranging from 1.5kW to 15kW.  Depending on the power level the units have different input voltages, covering most of the global input voltages.

GH1.5kW / G1.7kW:                     1ø 85 to 265Vac
G2.7kW / G3.4kW:                       1ø 170 to 265Vac or 3ø 208Vac or 3ø 400Vac
G5kW / GSP10kW & 15kW:        3ø 208Vac, or 3ø 400Vac or 3ø 480Vac

Ensure that the manufacturer has internal fuses fitted, as some low cost power supplies do not.  A high voltage fuse is required for each phase.  They are bulky and are not inexpensive.

After reading this blog, you might even take a second glance at those big grey mystery boxes surrounded by chain link fencing and high voltage warnings in the company parking lot!

Power Guy

Monday, September 30, 2019

Fan cooling power supplies, which airflow direction is better?

Even though the efficiency levels of new power supply designs are now routinely in the 94 to 95% range, the push for higher power densities continues. Fan cooling is one option to reduce the product’s overall size for mid to high power requirements. Should the fan blow air into the power supply or extract air out?  This depends on several factors.

In rack mounted products like the Genesys+ programmable power supplies the airflow is drawn through the front and out the rear. This is to offer the best possible cooling for these products, to optimize the overall system’s performance and avoid the operator being subjected to hot exhaust air.


For enclosed power supplies, air flow direction can depend upon which direction the system air is being directed.  Having the power supply’s air flowing in the opposite direction to larger system’s fans can cause the airflow to reduce dramatically due to the system’s pressure, and cause overheating.

Where the fan is positioned in a power supply is another consideration.  The lifetime of an electrolytic capacitor is extremely sensitive to heat.  Each 10oC rise in the capacitor’s temperature will halve its operating life, thus cool air has to be directed accordingly.

Figure 1 shows the top view of a typical product (the cover removed) and the location of the capacitors and fan.  In the case of this power supply, as the input and output connectors are both located on the left side (front) for system wiring access, the fan is situated on the right hand side (rear).


Figure 1: Electrolytic capacitors and fan location


Should the fan blow air out, or in? As with any design, there are advantages and disadvantages.

Figure 2 shows the fan blowing air out (exhaust). 





Figure 2: Fan exhausts the hot air
Advantages

The cool air is drawn in over the output capacitors, this keeps these components cool and their lifetime is improved.

As indicated by the size of the arrows, the speed of the air entering the power supply is lower than the exit speed.  This is due to the cross sectional area of the input being twice that of the output  Lower speed air is less likely to draw in outside contaminants (dust and dirt) which could impact product lifetime.

Disadvantages

The input capacitors receive warmer air, but with an air directing baffle and perforations in the cover this could be mitigated. In general though, the input capacitors are less sensitive than the output filtering capacitors.

Higher speed airflow cannot easily be directed at hot items like magnetics.

Hot air is drawn across the fan bearings, which could affect fan life. If the fan speed is controlled according to ambient temperature, this too would be mitigated. Higher quality or higher temperature fans can also be used.



Figure 3 shows the fan blowing cool air in.


Figure 3: Fan draws in air



Advantages

Cool air is drawn across the fan bearings, increasing fan life.
Fast moving cool air creates backpressure and can be directed at hot areas, like the magnetics, reducing the overall de-rating of the power supply.

Disadvantages

The output capacitors may run hotter.  Larger capacitors can be used, which will have less internal heating and run cooler.
More contamination may be drawn into the power supply.

Many fan cooled power supplies offer “reverse fan” options on their datasheets. Often due to reduced thermal performance within the supply and heated air moving through the fan, additional derating may apply.

Power Guy







Friday, August 30, 2019

When are fuses needed in Line and Neutral for industrial applications?



Last month I wrote about why using a dual input fused medical/industrial power supply might cause an issue in some industrial applications.   Shortly after publication a customer requested an application note showing the fitting an external fuse in the Neutral connection of an industrial DIN rail power supply application.  DIN rail power supplies are rarely certified to the medical safety standards and usually only have a single input fuse.

Upon investigation, the application was in this case operating a power supply phase to phase from a 3-phase WYE configuration in North America, see Figure 1.

Figure 1: Phase to phase connection (208Vac)

Connecting phase to phase enables the equipment to be supplied with 208Vac which draws less input current than using 115Vac.  This allows the use of smaller wire gauges and connectors which saves money and are easier to install.  The power supply is connected as shown in Figure 2.


Figure 2: Power supply connected phase to phase

In the UL safety report Conditions of Acceptability, or Technical Considerations section, reference is made to all testing being performed on a protected branch circuit rated for 20A; this section also lists the items that a user should consider before applying power to the unit.

If a short or overload inside the power supply occurs across the Line and Neutral or Line and Ground, the internal fuse F1 will open.  If the short occurs from Neutral to Ground, the fuse or breaker at the distribution panel will open.

If, however, the equipment connection to the building installation wiring is made via a non-industrial plug and socket, then an external fuse or breaker (F2) has to be added in the Neutral line as shown in Figure 3.  This applies to input voltages of less than 240Vac +10%.


Figure 3: The installation of an external fuse or breaker in the Neutral

Always have your equipment installed by a qualified electrician and checked by a safety engineer for compliance to the relevant building and electrical codes!

Power Guy

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