What are Bi-polar power supplies?

 Most power supplies are “unipolar”, capable of suppling a positive voltage and current only (Figure 1).

Figure 1: Single quadrant unipolar power supply

In some applications the load may need the power source to provide a positive or a negative output voltage. One application for this would be a DC motor, applying a positive voltage would make it rotate clockwise and a negative voltage counterclockwise.

One could use a bipolar power supply (Figure 2), transitioning seamlessly between providing a positive or negative voltage from a single pair of output terminals.

Figure 2: Dual quadrant bipolar power supply

As these types of power supplies are complex and do not have the economy of scale of a unipolar, they are usually more expensive and niche. The term “bipolar” can also refer to four-quadrant power supplies which can source and sink power (Figure 3).

Figure 3: Four-quadrant bipolar power supply

The simplest way is to buy a power supply with a polarity reversal switch. In the case of the TDK-Lambda PHV series of programmable high voltage power supplies, this is an optional feature. It can be changed electronically for voltages up to 35kV and manually at up to 65kV. One drawback is that the power supply may have to be turned off during the change which will extend test times.

TDK-Lambda PHV power supply

An alternate way is to use polarity changing relays. TDK-Lambda’s Genesys+ and Z+ series of programmable power supplies have two programmable terminals. These can be electronically programmed to active relays as shown in Figure 4. With the relays off, a positive voltage will be provided to the load and when the relays are turned on, a negative voltage will be applied.  Note that if remote sense is being used (+s and -s), those connections will have to be changed too. The output will have to be disabled during the change-over, again extending test times.

Figure 4: Using relays to change the polarity of the power supply voltage

TDK-Lambda Genesys+ power supply

TDK-Lambda Z+ power supply

Power Guy

Voltage adjustment methods for power supplies and converters

Introduction

AC-DC power supplies and DC-DC converters often feature an output voltage adjustment, the range of which is stated in the datasheet. An AC-DC power supply may have a potentiometer accessible to set the output voltage from the original factory set point. (See Figure 1). Adjusting the output voltage may be needed to compensate for voltage drops in the load cables, or to optimize the load’s performance or efficiency.

Figure 1: RWS50B output adjustment potentiometer (left side)

 

Board mount DC-DC converters may also have an output voltage adjustment range, usually achieved via the “trim” terminal. The trim connection is either pulled high or low via a resistor depending on the product.

 

Isolated DC-DC converters

 

An isolated model typically has a narrow adjustment range. For example TDK-Lambda’s 30W rated single output CCG30 models (Figure 2) can be set within + or -10% of the nominal output voltage.

Figure 2: CCG30 DC-DC converter

 

A resistor is fitted between the trim pin and either the -Vout or the +Vout terminal depending if the voltage needs to be increased or reduced (Figure 3).  If no resistor is fitted, the output will be the nominal value stated on the datasheet.

 

 

Figure 3: Adjusting the output of a CCG converter

 

Non-isolated DC-DC converters

 

Non-isolated DC-DC models can have very wide adjustment ranges because of the topology used in the design. TDK-Lambda’s i7C4W008A120V converter (Figure 4), for example, has an output voltage range of up to 9.6 to 48V. This enables one part number to be used for several different system voltages and help inventory management.

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

 

The trim connection for the i7C, and other non-isolated models like the i3A, i6A and i7A, is slightly different.  If no trim resistor is fitted the output voltage will default to the low limit. From Figure 5 the trim resistor is always fitted between trim and –Vout. The full product specification provides the values and formula.  As the resistance is lowered, the output voltage increases. 

Figure 5: i7C trim circuit

 

Adjusting a converter using a digital-to-analog converter

 

With the growing number of autonomous applications for battery powered robots and drones, there are demands for the output voltage to be electronically adjusted using a programmable digital voltage.  A lower voltage might be used to extend battery life, enabling the robot to return to its charging station for example.

 

One method is to use a D/A converter (DAC) connected to the trim terminal of the DC-DC converter (Figure 6). TDK-Lambda recently posted an application note on how to use a DAC to adjust their buck and buck-boost non-isolated series. A link can be found here.

 

Figure 6: Typical application circuit for digital output voltage adjustment

 

The application note details the type of DAC needed and shows the test results of an i7C converter being digitally controlled.

 

The DAC must be connected with short, direct traces to trim and ground terminal of the DC-DC power module. To avoid regulation errors from voltage drops, the ground paths need to be kept separate from the power traces carrying load currents. Any noise or voltage drop at the DAC output will cause unexpected output voltage variation.

 

It should also be noted that when adjusting the output up in voltage, the maximum power rating of the power supply or converter is not exceeded. Likewise the maximum current must not be exceeded when the voltage is below the nominal set point.

 

Power Guy

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

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

What is UKCA? Updated Sept 2nd 2021

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 1^st 2021 but to give businesses time to adjust (in most cases) the CE marking can continue to be used until January 1^st 2023 (was 2022).  From that date onwards, CE marking will no longer be recognized in Great Britain.

 

Until January 1^st 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 1^st 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

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 40^oC ambient, or 5 minutes with 115Vac at 30^oC. 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