Tuesday, June 29, 2021

Charging capacitors using constant current power supplies

We have all seen defibrillators in movies and on TV. The device is switched on, the paddles are applied to the patient’s chest, the operator shouts “clear” and a jolt of electricity, around (3,000V) is applied to the patient. If a second application is required, a short period of time is needed to recharge the defibrillator. Similarly, professional photographers often use studio strobe lighting, which requires a delay or “recycle time” to recharge after a photograph is taken.

Many pulsed load applications use capacitors to store energy. This enables high levels of current to be available to a load for a very short duration. The capacitor should be situated next to the load to provide a low impedance source. A power supply (or battery for portable equipment) is used to charge the capacitor to a set voltage.  There are two ways of charging a capacitor, using a fixed voltage power supply or using a supply that is capable of providing a constant current.

Lasers are now commonly used in cosmetic surgery equipment, material cutting and additive manufacturing (including 3D printing). Many lasers do not operate in a continuous-wave mode, but are pulsed on and off at extremely high frequencies to control the amount of heat energy they apply to the material.

Using an off-the-shelf constant voltage power supply to charge a capacitor can cause problems. When the power supply is initially connected to the capacitor, it will try to deliver its maximum allowable current and probably go into an overload condition. An uncharged capacitor is effectively a short circuit to a constant voltage power supply and if its protection circuit is the hiccup type, it may remain locked in that state. It is also not wise to repeatedly operate a power supply in an over-current condition for reliability purposes.

One method of avoiding an overload condition is to put a resistor in series with the capacitor to limit the current (Figure 1).


Figure 1: Resistor limiting the capacitor charging current

The main drawbacks to this approach are potentially significant power losses in the resistor and the charging voltage has an exponential response time. This is defined by:

Vcap = Vin x (1-e(-t/RC))

The term RC “time constant” is used. This is the time required to charge the capacitor, through the resistor, from a discharged state to approximately 63.2% of the value of the applied DC voltage.

Figure 2 shows an example of a 10,000µF capacitor (C) charging up to 2,000V via a 100Ω resistor (R).  After 5 time constants the capacitor is approximately 99% charged. In our case the time to charge would be 5RC: 5 x 100 x 0.01 = 5 seconds.


Figure 2: Capacitor charging curve for a 2,000V 10,000µF capacitor via a 100Ω resistor

Another method is to use a constant current power supply. Note, we do not need a series resistor as the power supply will internally limit the amount of current supplied (Figure 3). This current level is usually user adjustable.  Charge efficiency is dramatically improved with no losses in the resistor.


Figure 3: Constant current power supply connection

In this case (Figure 4) we do not have an exponential rise, but a controlled linear increase in voltage until the capacitor is fully charged. The amount of time to charge the capacitor is determined by the power supply. One supply with twice the output current will halve the charging time.

 

 

Figure 4: Charging a capacitor with a constant current power supply

Once the desired capacitor voltage is reached, the power supply will stop delivering current.

For someone who is very familiar with constant voltage power supplies, constant current power supplies are a little like driving on the “wrong” side of the road in another country. TDK-Lambda’s High Voltage Product Line Manager had sent me an application story which took me a couple of reads before I understood the problem!

A customer had a 1,000V rated ALE 802 high voltage supply with a charge rating of 18A. To test it, an 80nF capacitor with a 20MΩ resistor in series was connected across the output and the power supply shutdown. As the power supply can deliver an 18A constant current it would try to generate 18 (A) x 20,000,000 (Ohms) = 360,000,000V (V = I x R). Of course the power supply protected itself and shutdown! Even if the resistor was shorted out, 18A charging up the very small 80nF capacitor would take just 4µs (charge time = C x V/I). That was faster than the power supply’s control circuit.

His words of wisdom to the customer were “First, keep the voltage drop in any series resistance/impedance to around 1% or less of the rated voltage to avoid false overvoltage issues. Second, use a capacitor that charges in 500microseconds or more to give the control electronics sufficient time to respond. There’s always exceptions to the rules, but these numbers are a good starting point.”

TDK-Lambda has a large range of programmable power supplies that can operate in constant current mode. The 4000W TPS4000 3-phase has nominal output voltages of 24V and 48V. At the other end of the spectrum is the ALE series, capable of providing outputs from 0 to 50,000V with power levels up to 1MW. The fully programmable GENESYS+™ series of constant voltage/current power supplies have 0 to 10V up to 0 to 1500V outputs with power levels from 1,000W to many tens of kWs.


Figure 5: TPS4000 and ALE power supplies

There is a great selection of information on high voltage capacitor charging on TDK-Lambda’s website, including a pdf with useful equations.  https://product.tdk.com/info/en/products/power/tec_data/ps_ale.html

Power Guy


Friday, April 30, 2021

What do buffer modules do and how do I use them?

Introduction

When the AC input to the power supply is interrupted for a period longer than the power supply’s hold-up time, the addition of a buffer module will continue to provide power to the load until its stored energy is depleted.  Without a buffer module, short interruptions of greater than 10 to 20ms may cause a system to reboot, or leave a machine (like a robotic arm) in an undesirable state or position; hence the term “buffer module”, as it “buffers” the load from interruptions of the AC power.

An uninterruptable power supply (UPS) could be used; however it would consume more energy, cost more to operate, and add considerable cost to the system. As batteries are normally used to store the back-up energy, periodic preventative maintenance and battery replacement is required.  Could a bank of capacitors be attached across the output of the power supply?  Yes, maybe, but I suggest you read a previous blog article outlining the pros and cons of doing so.

How do I connect them?

The buffer module is connected in the same way as additional capacitors would be, across the power supply / load.  See Figure 1.

 

Figure 1: Connection of a buffer module

What will they do?

Current will only flow into the buffer module when it needs charging. It will only flow out (discharge) to the load when the AC input is interrupted and the output voltage of the AC-DC power supply to starts to decrease.

Figure 2 shows that without a buffer module, the voltage to the load will drop once the power supply’s hold-up time “T” is exceeded, typically within 10 to 20ms.

 Figure 2: No buffer module in circuit

With a buffer module in circuit (Figure 3), the voltage to the load is present during a brief loss of the AC. Time “T” is the power supply’s hold-up and time “Tb” is the additional hold-up provided by the buffer module. The maximum value of Tb will depend on the energy shortage capability of the buffer module and the amount of load being drawn.

 

Figure 3: Buffer module in circuit

The extended hold-up from the buffer module can also be used to provide power to the load for a longer period of time. This may allow a piece of machinery to safely shutdown or store information on the progress of a particular process.  Figure 4 shows this.

Figure 4: Keeping the load powered longer

The time Tb is stated on the manufacturer’s datasheet at a specified power level.  The 20A rated TDK-Lambda DBM20 for example provides an additional 250ms at a power level of 448W.  When operated at lower power / current levels, the hold-up increases, see Figure 5. 

Figure 5: DBM20 buffer time vs load current

If the load is 5A, the buffer time (Tb) will be just under 1 second. Buffer modules like TDK-Lambda’s DBM20 and ZBM20 utilize high quality electrolytic capacitors to store the energy.  This eliminates the use of rechargeable batteries and any necessary servicing or periodic maintenance. The curves labelled as “typical” include the tolerance in the electrolytic capacitors.  Buffer modules can usually be connected in parallel to further extend the time Tb.

Fixed Mode and Variable/Dynamic Mode

These terms can be seen on some datasheets and a switch found on the buffer module (Figure 6).

Figure 6: DBM20 showing the fixed mode and variable / dynamic mode switch 

When the switch is in “Fixed Mode” the output voltage will be set at the factory setting. For our DBM20 that is 22.4V for use with a 24V AC-DC power supply. When the AC is interrupted, the buffer module will start supplying power to the load at 22.4V.   This provides hysteresis to stop the buffer module from discharging unnecessarily. Note in Figure 3 and 4, the output has dropped slightly (24V vs. 22.4V).

If the switch is in the “VIN-1” position, the buffer module will operate in “Variable” or “Dynamic” mode.  VIN-1 refers to the input voltage (VIN) minus 1V.  If the output voltage of the AC-DC power supply is set to 28V, then the buffer module will start supplying power to the load at 27V. The manufacturer’s datasheet will state the input voltage range when used in this mode.

The TDK-Lambda ZBM20 module mentioned in this article is a similar product suitable for non-DIN-rail mount applications.

Power Guy

Friday, February 26, 2021

What is UKCA?

Before I start this article, I will clarify some geographic terms.  The United Kingdom (UK) consists of four countries, England, Scotland, Wales and North Ireland.  Great Britain (GB) consists of England, Scotland and Wales.  This is why you may have heard the phrase “Great Britain and Northern Ireland” or the “United Kingdom of Great Britain and Northern Ireland”.  Under the Brexit (withdrawal of the UK from the European Union) agreement though, Northern Ireland is currently following EU (European Union) regulations to ease border checks with the Republic of Ireland.


According to the UK government website, “the UKCA (UK Conformity Assessed) marking is a new UK product marking that is used for goods being placed on the market in Great Britain (England, Wales and Scotland). It covers most goods which previously required the CE marking.”  Figure 1 shows the marking.



Figure 1: UKCA mark

 

Before Brexit, the UK used the CE Mark to show conformity with the relevant EU legislation. After Brexit, UK and EU legislation may start to differ, and the UK needed an independent mark to indicate conformity with UK law. In the future, products will need to carry both marks if imported into both areas.

 

Some important points:

 

Note that this is a label change process not a safety certification process

 

The UK currently references the same EN standards as the EU - for a power supply these include EN 62368-1, EN 60335-1 and EN 60601-1. This means there is no additional safety certification work required.

 

The UKCA marking came into effect on January 1st 2021 but to give businesses time to adjust (in most cases) the CE marking can continue to be used until January 1st 2022.  From that date onwards, CE marking will no longer be recognized in Great Britain.

 

Until January 1st 2023 there is an option to apply the UKCA mark to the product, product packaging or an accompanying document. This can also be a packing slip and/or an invoice.

 

From January 1st 2023 onwards, it is compulsory to have the mark on the product and if this is not possible, the mark should be on the product packaging or accompanying product documentation as appropriate.

 

The product, product packaging or an accompanying document must also have the contact address of the authorized UK representative. In TDK-Lambda’s case this will be TDK-Lambda UK.

 

The UKCA marking is not a replacement for the CE marking. Both marks will be required if a product is to be sold in both regions. Both will require a Declaration of Conformity.

 

For further advice and application support on this, or any other power supply related topic, please contact your local TDK-Lambda office.

 

 

Power Guy

Monday, December 28, 2020

Peak power ratings with convection and conduction cooled power supplies

Many open frame power supplies now offer two power ratings, one with convection or conduction cooling and one where external forced air is applied.  As some applications require low acoustic noise levels, for example in healthcare where patient comfort is important, fan-less cooling is preferred.  This can be achieved through operating a power supply at its convection rating, and if it is also capable of conduction cooling, a combination of both.

If the output load significantly varies with time it may be possible to utilize a power supply with a peak power rating.  This allows a lower power product to be selected, saving cost and reducing its size, as the convection cooled peak power rating is often the same as the forced air continuous rating. 

For example, modern hospital beds are fitted with electric motors that can adjust the angle of the bed to allow the patient to sit upright, or raise the lower portion to elevate the legs. These motors may only operate for short periods of time. The same will apply for dental chairs, thermal warmers for babies and incubators.

Power supply manufacturers will state the maximum peak power level and the length of time it can be drawn.  The datasheet or application notes indicate if the peak power rating is calculated using RMS or average power. Alternatively, examples may be given in the application notes or instruction manual with a list of components to be monitored and their maximum temperatures.

TDK-Lambda’s CUS600M power supply (Figure 1) has a convection cooled and RMS peak rating of 400W. It can deliver 600W peak for a maximum continuous duration of 10 seconds.



Figure 1: TDK-Lambda’s CUS600M power supply 

To determine the RMS power of a repetitive peak load, we shall use the example shown in Figure 2. OPa = minimum output power drawn, OPb = maximum output power, t = peak load time in seconds and T = quiescent load time in seconds.

 


Figure 2: Repetitive peak power load example

To calculate the RMS power we use equation 1.

Equation 1:  RMS Power = √((OPb2 x t + OPa2xT)/(t + T))

It OPb=600W, OPa=100W, t=10s and T=20s, the maximum RMS power would be 356W.


Alternatively, if the power supply had an average power rating stated we would use equation 2.

Equation 2:  Maximum Average Power > ((OPb-OPa) x t/T) + OPa

For the previously used values that would calculate as 350W.

TDK-Lambda’s CUS400M power supply (Figure 3) can deliver 400W with forced air or, with a combination of convection and conduction cooling, provide up to 250W continuous output power and a peak power of 400W.

Figure 3: TDK-Lambda’s CUS400M power supply

Derating curves (Figure 4) are provided in the application notes as a guide rather than firm limits with the CUS600M, as it is expected that component temperatures will be measured and confirmed in the end system.


Figure 4: CUS400M peak power performance curves

Whereas many power supplies have peak power durations measured in seconds, the mechanical construction of this series provides additional cooling through the use of a metal baseplate. With a 230Vac input, the 400W peak rating applies up to 30 minutes with a 40oC ambient, or 5 minutes with 115Vac at 30oC. This makes this type of product very suitable for powering applications with occasional peak demands.

To ensure long term reliability, convection cooled power supplies should always have their critical component temperatures measured to ensure conformity with the manufacturer’s recommendations. This should be done in the end equipment at the worst-case conditions of ambient, input voltage and orientation, allowing the temperatures to fully stabilize.

For further advice and application support on this, or any other power supply related topic, please contact your local TDK-Lambda office.

Power Guy 

Monday, October 26, 2020

Using a Class I power supply in a Class II construction

I have to admit that I was a little skeptical when I was first told it was possible to use a Class I power supply (with an earth ground connection) in a Class II (no earth ground connection) construction.  After some discussion with a colleague, he pointed out that if certain actions are taken, it is perfectly safe - even in a medical application.

A general statement of safety is that a product must have two levels of protection between live (primary) parts and the end user.  If one level of protection fails, the end user is still safe.

A Class I example would be a clothes dryer. Usually they have an earth ground connection which is securely connected to the drier’s metal enclosure.  This is to protect the user if the insulation on the AC input cable frays or the heater element malfunctions and makes contact with the enclosure.  The fault current would flow through the earth wire, tripping the home’s earth leakage breaker and the AC power would be disconnected.

For our clothes dryer example, one protection level is the insulation on the internal wiring, the second level is the protective earth connection.

As mentioned in previous blog articles, the home healthcare standard (IEC 60601-1-11) stipulates the need for Class II equipment, as the integrity of the safety earth connection cannot be guaranteed in the home environment.

TDK-Lambda’s medically certified open frame CUS600M series (see Figure 1) can be used in either Class I or Class II applications.  It has two levels of protection, achieved through the use of double or reinforced insulation.



Figure 1: CUS600M power supply

 

The CUS600M/EF versions though have a metal enclosure and fan fitted. See Figure 2.  This changes the situation for a Class II application as during a single fault condition, the metal cover may  become live. If the cover was accessible to the user, we would no longer have two levels of protection. Fortunately, there is a solution!

Figure 2: CUS600M/EF power supply

 

First, the power supply must have 1 x MOPP (Means of Patient Protection) from the input to ground (chassis) and 1 x MOPP from the output to ground. 


Our CUS600M/EF has that - see Figure 3.


Figure 3: CUS600M/EF MOPP isolation barriers

Second, ensure the output touch current does not violate the safety requirements of the end equipment.


In our case the CUS600M touch current (enclosure leakage) is very low, <100µA, which should be acceptable in most applications.

Third, the power supply chassis should have 1 x MOPP isolation from the final equipment enclosure.

From Figure 4 it can be seen that we now have achieved our two levels of protection, even without the use of an earth ground connection.



Figure 4: CUS600M/EF isolation from the final enclosure

The 1 x MOPP isolation between the power supply and the system enclosure can be achieved by mounting the power supply on a plastic / non-conductive insulating plate or by mounting the unit on plastic / non-conductive stand-offs. See Figure 5.


Figure 5: Power supply with a metal chassis mounted on plastic / non-conductive stand-offs

Last, the EMC performance must not be adversely affected


The EMI performance of the CUS600M open frame power supply in a Class II application already meets EN55032-B for both conducted and radiated emissions.  The same applies to its immunity performance.

 

Even a Class I modular power supply like QM series can be used in this manner too, as the output to chassis isolation is 1 x MOPP. It should be noted that many other modular power supplies just have basic (non MOPP) isolation.

For further advice and application support on this, or any other power supply related topic, please contact your local TDK-Lambda office.


Power Guy

Friday, August 28, 2020

Buck & Buck-Boost Converter Operating Ranges

This is a follow-on to a recent post where I discussed the difference between a buck, boost and buck-boost converter.  Now we will review the operating ranges of a buck and a buck-boost converter.

As a quick recap - buck and buck-boost DC-DC converters are widely used in power supply designs, and have been for many years.  They are popular because of their simplicity, low cost and high efficiency.  There is no transformer used in the design, and as such, there is no isolation between the input voltage and the output voltage.  Manufacturers like TDK-Lambda offer both buck and buck-boost converters.  With less components and complexity, a buck converter will offer a lower cost, higher efficiency and either a smaller package or more output power than an isolated converter.

As with any power supply and DC-DC converter, there are limits to their operation.  A programmable power supply like TDK-Lambda’s GH10-150 is rated to operate from 0 to 10V with a maximum current of 150A with a maximum output power of 1500W.  It is rather unreasonable to expect it to deliver 5V at 300A.

The main restriction of a non-isolated (step down) buck DC-DC converter is that the input voltage has to be higher than the output voltage. One example is the i6A4W010A033V-001-R which has an input range of 9 to 53Vdc, an output adjustment range of 3.3 to 40V and a maximum output current of 10A.  Figure 1 shows the relationship between the input voltage and the allowable output voltage.  Only the maximum output power restricts the amount of output current that can be produced.

Figure 1: i6A4W 10A buck converter output versus input voltage

A non-isolated buck boost (step-up / step-down) DC-DC converter has different constraints to its operation, besides the rated output power, current and voltage.  The TDK-Lambda i7C series has quite clear operating parameters on its datasheet, so we will use that as an example.

The part number i7C4W008A120V-001-R has an input voltage range of 9 to 53V, an output adjustment range of 9.6 to 48V and a maximum output current of 8A. In the datasheet the operating range is defined by two plots. These show the valid input and output voltage operating range and the maximum output current against input and output voltage.

Figure 2 is a pictorial representation of the valid input and output voltage operating range stated in the datasheet.  Note this differs significantly from the buck converter discussed previously, as the buck-boost topology allows it to generate voltages lower and higher than the input voltage.


 

Figure 2: i7C 8A valid input and output voltage operating range

From Figure 3, we can see the x axis matches the 9.6 to 53V input range stated on the datasheet and Figure 2.  The y axis has a maximum limit of 8A for the output current. There are four plots for different output voltages.


Figure 3: i7C 8A Maximum output current vs the input and output voltage

As an example, a machine has 24Vdc available from an existing AC-DC DIN rail power supply. A new feature, which requires 48Vdc at 3A is required.  One way of generating this additional output voltage is to use a non-isolated DC-DC converter, like the i7C4W008A120V-001-R. We would use the purple plot (48Vo) and see that the maximum current we can safely draw from the converter is around 5A.  If the DIN rail power supply had a 24V battery back-up feature and the AC power failed, the i7C would continue to deliver 48V at 3A until the battery discharged down to 15V.

When a buck-boost DC-DC converter has a low input voltage, the input current switched by the FET is much larger than when it is operating at a high input voltage. This is the main restriction to the amount of output power it can deliver.

Like any power supply or DC-DC converter, having determined that the electrical parameters have been met, proper thermal management then has to be followed to ensure long term, reliable operation.


Figure 4: TDK-Lambda’s i7C 300W buck-boost DC-DC converter

 

Power Guy

Friday, July 31, 2020

What are buck, boost and buck-boost DC-DC converters?

Buck, boost and buck-boost DC-DC converters are widely used in power supply designs, and have been for many years.  They are popular because of their simplicity, low cost and high efficiency.  There is no transformer used in the design, and as such, there is no isolation between the input voltage and the output voltage.

 

In this post I will explain the differences between these three converters and a high level review on how they function..(As a note the simplified schematics do show diodes and switches, in reality FETs are used as synchronous rectifiers to reduce losses and improve efficiency.)

 

A buck converter reduces voltage and the output voltage is lower than the input voltage. See Figure 1.


Figure 1: Buck converter

 

When transistor S is turned on, energy is stored in inductor L as the current flows to the load and capacitor C is charged. When S is off, the energy stored in L is released and current flows into the load and circulates via diode D. Capacitor C also provides energy to the load.  This is repeated at a high frequencies, greater than 100,000 times a second.  The length of the time S is turned on defines the output voltage.

 

A boost converter increases voltage and the output voltage is higher than the input voltage. See Figure 2.



Figure 2: Boost converter

 

When transistor S is turned on, current flows through inductor L, through transistor S back to the input. During this period energy is stored in the inductor.  When transistor S is off, the inductor acts a voltage source in series with the input voltage.  The inductor’s stored energy is circulated through diode D to the load. This charges capacitor C to a higher level than the input voltage. Again, the length of the time S is turned on defines the output voltage.

 

This boost converter topology is also used in most Power Factor Control (PFC) sections of AC-DC power supplies. The control IC is different of course, as its purpose is to ensure the AC input current drawn is sinusoidal in shape.  At high line voltages greater than 240Vac the DC input may be higher than the voltage on capacitor C.  This will reduce the PFC boost converter’s performance and the power factor will be degraded slightly.

 

A buck-boost converter is a combination of a buck and boost converter. The output voltage can be higher or lower than the input voltage. See Figure 3.



Figure 3: Buck-boost converter

 

As you can see the circuit is more complicated and has more components.  S2, L, and D2 is the boost converter (S1 being on) and S1, L and D1 the buck section (S2 being off).

 

Many manufacturers like TDK-Lambda offer both buck and buck-boost converters.  With less components and complexity, a buck converter will offer a lower cost, higher efficiency and either a smaller package or more output power.

Figure 4: TDK-Lambda’s i7C 300W buck-boos tDC-DC converter

 Power Guy


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