Or: How to build a polarity switching power source without using a bipolar power supply

Welcome to the second instalment of “DC Power Supply Secrets”, a series in which I share engineering hacks and fun facts about DC power supplies.

Let us consider the PBZ line of intelligent bipolar power supplies, which are known for their rapid operation and wide range of features (Photo 1). Unfortunately, the PBZ’s functionality and performance comes at a price. Many clients tell me that while they recognize the quality of the PBZ, they don’t have the budget to buy one. At Kikusui, we are dedicated to providing a range of specialist devices that caters to our clients’ demands for specification and function. Because our sales volumes will never be on a par with consumer electronic manufacturers, however, we do not enjoy the benefits of economies of scale.

That said, I do not believe that companies in our position should simply take the approach of forcing high added-value products on their customers. In fact, Kikusui makes a point of offering its customers options for “next-best” and alternative solutions suitable for the intended application and the degree of precision testing required.

One example of such an alternative solution is the basic polarity switching power supply system discussed here. This is a solution I proposed to a client who said they wanted to perform polarity switching tests on current sensors but wanted an alternative to the PBZ series which they could not afford.

This polarity switching power supply system assumes a required output of 10 V/±100 A. While the high-performance device that meets this specification (the PBZ SR series) has a standard retail price of around 40,900 dollars (4.5 million yen) including tax, the alternative solution discussed here costs less than half of that.While this solution obviously comes with a few restrictions in terms of performance and operation, I believe it represents good value for money if it matches your requirements.

Overview and composition of system.

The system I discuss here combines DC power supplies with an electronic load device and was proposed by me in response to a request from a client to come up with a basic system built from standard components that could be used for testing current sensors and could supply both positive and negative current without using a bipolar power supply or polarity switching device.

By using DC power supplies (the PWX1500L) to supply the positive and negative current combined with an electronic load (the PLZ1004W), the system is able to supply a current sensor with both positive and negative current (at 100A) without using a switching device (Figure 1).

How it works.

The DC power supplies are operated in continuous voltage (CV) mode in which the current is regulated not by the DC power supplies but by the electronic load. The DC power supplies on both the positive and negative side of the circuit are set to continuous voltage (CV) mode and output current is set at no less than the minimum operating voltage of the electronic load plus the voltage drop across the current sensor (PL Z1004W: at least 1.5 V +α). The electronic loads on both the positive and negative side of the circuit are set to constant current (CC) mode and configured to supply the desired current.

In order to output positive current, the user sets the desired current and activates the electronic load on the positive side of the circuit while deactivating or setting to 0A the electronic load on the negative side of the circuit. In order to output a negative current, the user sets the desired current and activates the electronic load on the negative side of the circuit while deactivating or setting to 0A the electronic load on the positive side of the circuit.

Note that if current is applied to the electronic loads on both the positive and negative sides of the circuit simultaneously, the current flowing across the current sensor will be the sum of these two currents.

Users should be aware that this system creates a lag when polarity is switched. Unlike a bipolar power supply, it is unable to transition smoothly from positive to negative output without switching off. The switching lag is in the order of a few hundred milliseconds and varies depending on the method used to control the electronic load (Figure 2).

Sample application.

We created an experimental circuit that had the same configuration as that shown in Fig 1 but used a shunt resistor in place of the current sensor (Figure 3). The waveform of the current flowing through the shunt resistor, as measured with a current probe, is shown below. We tested the circuit with currents of ±5A and 1A, 3A and 5A by manually configuring the control panel on the electronic load.

• Waveform at ±A current (Figure 4)
  Positive/negative side DC power supply: set at CV10V output on
  Positive/negative side electrical load: set at CC 5A: load on/off

• Waveform at ± (1A, 3A, 5A) current (Figure 5)
  Positive/negative side DC power supply: set at CV10V output on
  Positive/negative side electrical load: set at CC 1A: load on: 3A/on: 5A/off

Cautions.

Users of this system need to be aware of the following:

(1) Even when a load is deactivated or set to 0A there will be some current leakage from the current sensing resistor in the electronic load. The magnitude of the leaked current (in the order of several hundred kΩ) depends on the specification of the electronic load.

(2) The COM terminal of the electronic load cannot be connected to the DC supply on either the positive or negative side of the circuit. If performing external analog control, all elements need to be isolated.

(3) If the DC power supply or electronic load on either the positive or negative side of the circuit is turned on without connecting a current sensor (i.e. the circuit is open), an inverse voltage will be induced in the DC power supply that is turned off. You should not therefore turn on either the DC power supply or electronic load device without connecting the current sensor.

(4) The precision with which the current may be controlled is dependent on the performance specifications of the electronic load. If controlling the device using digital signals you need to take into account signal lag.