Short handbook of current sensing in power DC supply lines – Part three

Technology News | June 11, 2014

By eeNews Europe

Introduction

We may need to connect in parallel simple diodes or bridge rectifiers. In the simplest case the paralleled devices should be from the same type. This approach has several advantages, e.g.:

we can use smaller and cheaper rectifiers
the individual switching currents of the rectifiers is smaller, the electromagnetic emissions can be smaller and the filtering of these emissions can be a simpler task.
the total circuit can be made more reliable. In the case that one rectifier is damaged, the other may take the load for some time and the system will continue to operate.
the cooling of the rectifiers/bridges may become simpler.

But the paralleling of rectifiers or bridges has several drawbacks, e.g.:

it requires the usage of equalization resistors.
if we wish to monitor the operation of each rectifier, we should use special interface circuits capturing the signal from the equalization resistors and sending that signal to the ADC.
the pcb design can be more complicated
the larger number of the rectifiers or bridges can take more real estate on the pcb
we have some voltage drop over the equalization resistors

If need be, we can connect in parallel filtering capacitors for the rectifiers in the mains power supply. We may use smaller capacitors. This approach has several advantages as discussed in the following comment.

The capacitors with smaller capacity usually are:

with higher working voltages
with smaller leakage currents
smaller size, better quality (lower parasitic components),
widely available, lower price, etc.

But the paralleling of filtering capacitors may require equalization resistors in order to make the charge and the discarded currents equal for all paralleled capacitors.

Here we will examine several solutions for monitoring the currents in the paralleled rectifiers and paralleled capacitors. That monitoring task is important because ideally the current load should be divided equally between all paralleled rectifiers. Also that is important because we can monitor the operations of the components in rectifying blocks and to monitor the quality of the mains power supply.

Description of the circuits

Figure 3.1 presents a simplified circuit with paralleled rectifiers and paralleled filtering capacitors. Rd1, Rd2, Rd3 and Rd4 are the equalization resistors for the diodes.

Figure 3.1. Simplified circuit of paralleled rectifiers and paralleled capacitors

The differential amplifiers DA1 to DA4 capture the signals from these resistors and send them to the ADC. Rc1 and Rc2 are the equalization resistors for the paralleled capacitors. The amplifiers A1 and A2 capture the signals from these resistors and send them to the ADC.

The block circuits from Figure 3.1 may seem complicated but it gives the possibilities the monitor the status of all components in the rectifying section of the power supply.

Figure 3.2 shows the circuit of the differential amplifiers DA1 to DA4 from Figure 3-1.

Figure 3.2. Circuit of the differential amplifier capturing the signal from the equalization resistors for the diodes

The DAs amplify the signal from the equalization resistors in series of the diodes. The circuit consists of:

input dividers with R1, R2, R3 and R4
differential amplifier with diode protection,
filtering capacitors for low pass filtering C1, C2, C3 and C4.
Zener diodes D9 and D10 for limiting the output signal from the operational amplifier.

Some of the components in the circuits may be omitted if they not are necessary, e.g. if we do not need to reduce the input voltages we can omit R1, R2, R3 and R4.

The values of the components are calculated according to standard methods for these types of circuits. The gains of the differential amplifiers can be set to any appropriate value by R5, R6, R7 and R8. For an operational amplifier we may use single, dual or quadruple operational amplifiers depending of the case.

Figure 3.3 presents the circuit of amplifiers A1 and A2.

Figure 3.3. Circuit of the amplifier capturing the signal from the equalization resistors for the capacitors

This circuit amplifies the signals from the equalization resistors for the paralleled capacitors.

The operation of the circuit is similar to the operation of the circuit from Figure 3.3 and will not be described in details.

We may omit R3* if we do not need to reduce the input voltage for the OA. The gain of the stage is fixed with resistors R4 and R2.

Sometimes we may need to put in parallel rectifying bridges in order to obtain higher current for the load or to reduce the operating temperature for the bridges or for some other reasons.

Figure 3.4 gives a simplified circuit with paralleled bridged rectifiers and with paralleled filtering capacitors.

Figure 3.4. Simplified circuit of paralleled bridge rectifiers and paralleled capacitors

Rb1 and Rb2 are the equalization resistors for the rectifying bridges. The differential amplifiers DA1 and DA2 capture the signals from these resistors and send them to the ADC. Rc1 and Rc2 are the equalization resistors for the paralleled capacitors.

The amplifiers A1 and A2 capture the signals from these resistors and send them to the ADC.

The circuits of DA1 and DA2 are given on the Figure 3.3 and the circuit of A1 and A2 are given on Figure 3.4.

About the operational amplifiers

Most of the rectifiers in mains power supplies work at low frequencies, e.g. 50Hz, 60Hz, 400Hz, etc. The signals of charging and discharging the filtering capacitors are with similar frequencies (sometimes with double frequencies) and with relatively low slew rate. These signals are with irregular forms and have many harmonics but in most of the cases these harmonics are limited only to several kilohertz.

We will not examine the cases when we use much higher frequencies in the switching power supplies because they require different amplifiers and design considerations. But even in some of these cases we can use the amplifying circuits described here.

At low frequencies in the circuits discussed above and at low voltages we can use low cost general purpose OAs. The choice of the OA depends mainly on the power supply range, allowed input voltages for the OA, e.g.:

with bipolar power supplies we may use RC4558, MC1458, TL061/62/64, etc.
with single supplies we may use LM358, LM324 etc.

Usually the adjustment of the zero offset for the OAs for the described circuits is not needed.

The numbers of the pins of the OAs are not given in the proposed circuits in order to make them simpler.

Conclusion

This part 3 of the short handbook presents block circuits and detailed electrical circuits for current sensing in paralleled rectifiers and paralleled capacitors. The circuits are simple, low cost, reliable and efficient.

The monitoring circuits are built around operational amplifiers. The circuits can be adapted to large variety of applications. In case the rectifying sections create dangerous pulses and noise for other electronic components we should use appropriate protection components and protection layouts in the pcb for the operational amplifiers.

References and notes

Important and detailed information about the OAs and their applications can be found on the web pages and printed publications of the manufacturer and in the specialized magazines.

The data sheets of all components on the figures should be consulted.

Some of the abbreviations in the text:

ADC – analog to digital converter

AC – alternative current

CVCs – current to voltage converters

DA – differential amplifiers

DC – direct current

OA – operational amplifier

pcb – printed circuit board

SW – switch

Vz – Zener voltage

T, VT – Transistor