Handbook of current sensing in DC power – Part one
Introduction
The power supply is a vital part of every piece of electrical and electronic equipment. If it does not operate properly the entire piece of equipment or system will not operate or can be badly damaged.
The components in the power supply are exposed systematically to high surge currents and all sorts of perturbations from the AC mains power supply, from inverters, DC/DC converters, uninterruptible power supplies, switching on/off reactive loads, etc.
In order to obtain high reliability at acceptable cost we can use the paralleling of the components in the power supply. In order to evaluate the status of the components we should capture information about the currents, voltages and power dissipation of the most important parts in the power supply.
The current sensing circuits are applicable when we wish to monitor the current for driving peripheral loads in the electronic systems.
This handbook consists of three separate parts. Each part resolves a practical group of cases for current sensing. Those parts are listed below:
Part 1: Simple, low cost and robust high side sensing circuits for positive power supplies
Part 2: Simple circuits for sensing currents in negative and bipolar power supplies
Part 3: Simple circuits for sensing currents in paralleled rectifiers and capacitors
Part 1: Simple, low cost and robust high side sensing circuits for positive power supplies
Introduction
Frequently the MCU should have an idea about the power consumption from the positive DC power supplies in the embedded system. For example we should know approximately how the power consumption changes when:
* we switch ON and OFF particular peripheral blocks or devices of the system
* we change the mode of operation of particular parts of the system
* and how the peripheral devices as DC motors react to changes in the loads/environment, etc.
Usually the accuracy of these measurements is not very high and we can accept +-5% and sometimes less than that. One of the problems of these applications is that we may have need of rail to rail input and rail to rail output (RIRO) operational amplifiers (OAs). But these OAs may not have the appropriate power supply range, availability or may have other drawbacks. For example, the RIRO are usually limited to around 16V and sometimes well below that voltage or may not have enough drive capabilities, etc.
That is why here we will use bipolar transistors and popular OAs such as LM358, LM324, TL06x, TL08x, RC4558, MC1458 and similar amplifiers which can offer good solutions in many cases.
We will show several simple and low cost solutions for the DC positive power supplies because they are more frequently used. In fact the solution of the task consists of building an appropriate current to voltage converter (CVC) producing output voltage Vc proportional to the DC power supply current.
The output voltage Vc can be made bipolar or with single polarity with additional circuits or by adding offset voltage to the operational amplifiers.
Description of the circuits
Figure 1.1 shows probably the simplest non-isolated circuit for high side sensing in positive power supplies. The circuit uses, as a threshold detector, an amplifier with one PNP transistor without initial collector current. The capacitor C5 reduces the probability to have unwanted oscillation from the transistor in active mode.
Figure 1.1: Simplest circuit for high side sensing of positive power supplies using a PNP transistor without initial collector current.
The circuit has two ranges of the monitored current selectable with the switch SW1. The switch can be an electromagnetic relay or a manual switch. The voltage difference (Vin – Vout) is amplified by the transistor T1 and the amplified signal is available across the resistor R4. The signal can be used by an ADC after appropriate signal conditioning.
Some of the advantages of the circuit are:
* The circuit is operational and we may use it for approximate evaluation of the low, middle and even high voltages and all sorts of power supply currents (after using appropriate components).
* The circuit is very simple and low cost.
* We may connect the resistor R4 to the ground or to the positive voltage below Vin or to negative voltages according to need.
* When properly designed the circuit is difficult to damage.
* Calibration is not always needed because sometimes we need only to know if the output current is above a predefined value or when the current rises or falls when we activate or deactivate particular modules or when we change the mode of operation, etc.
Some of the disadvantages of the circuit are:
* The voltage drop over the current sensing resistors R1 and R2 can be significant, e.g. 0.5-1V depending of the transistor T1
* The circuit has a voltage threshold depending on temperature and the selected transistor T1.
The voltage threshold is changing with around -2.2mV/oC.
* The function between the load current and the voltage Vc is not linear.
* The reproducibility of the circuit is low; if we change the transistor we may need recalibration of the measuring circuit.
* If higher accuracy is needed we should do calibration procedure at several points (with several DC loads).
We can improve the circuit from Figure 1.1, e.g. we can make it more linear and less dependent from the changes of transistor T1.
Figure 1.2 shows the improved circuit for high side sensing of positive power supplies with analog amplifier and NPN transistor T1. The resistor R2 stabilizes the gain of T1. The gain of the circuit is approximately equal to –R3/R2.
Figure 1.2: Simplest circuit for high side sensing for positive power supplies with improved analog amplifier and NPN transistor with initial current.
The trimmer potentiometer P1 is used to adjust the initial current for the transistor T1.
If needed we can use the Zener diode D1 to limit the output voltage Vc. Vc is proportional to the current sensed by R1.
If we are in need of higher accuracy of the CVC we should use operational amplifiers (OAs) to build a current to voltage converter circuit with higher complexity. We may need to use general purpose OAs as LM358 or LM324.
The inputs and the outputs of these ICs can go approximately to ground level but cannot go to the positive power supply of the OAs – the minimal difference in that case is around 1.5V to the positive rail.
Fortunately, we can reduce the input voltages of the OA appropriately and after that we can amplify the difference in order to make good use of the OAs.
Figure 1.3 presents probably the simplest circuit for high side sensing for positive power supplies with a single differential amplifier without using a RIRO operational amplifier.
Figure 1.3: Simplest circuit for high side sensing for positive power supplies with single differential amplifier with not RIRO operational amplifier.
The resistor R1 produces a voltage proportional to the current for the load, e.g. if we have 0.1 Ohm and the maximum output current is 1A we will have voltage drop over R1 around 0.1V.
The resistors R2, R3, R4 and R5 reduce the input voltages of the OAs by two times and produce voltages within the working range of the OA.
OA works as differential amplifier with gain of ten (x10) and produces the voltage Vc which is proportional to the measured current of the load.
We can use any appropriate values of the resistors R1 to R10. The capacitors C5 and C6 may be removed if low pass filtering of the circuit is not needed.
Sometimes in equipment we have a low current DC/DC converter which can produce enough voltage and current for one additional LM358. The supply current of that IC is below 2mA at a power supply of 30V.
Figure 1.4 shows the circuit for high side sensing of positive power supplies with a single differential amplifier without a RIRO operational amplifier and with an additional DC/DC power supply using a 74C14.
Figure 1.4: Simple circuit for high side sensing for positive power supplies with single differential amplifier with not RIRO operational amplifier and with additional DC/DC power supply with 74C14.
The maximum input voltage +Vin is 15V and is limited by 74C14. The OA works as a differential amplifier with gain of ten. It amplifies the voltage drop across the current sensing resistor R1.
In this case the presence of the input dividers with R2, R3, R4 and R5 are not always needed.
We should pay attention because if the input capacitors C1 and C2 are discharged and the output capacitors C3 and C4 are charged, the voltage polarity across the current sensing resistor R1 will be inverted.
In the case when the input voltage +Vin is higher than 15V we may use a low power DC/DC converter with an operational amplifier and additional transistors or other appropriate DC/DC converter.
Figure 1.5 presents the circuit for high side current sensing for positive power supplies with a single differential amplifier without a RIRO operational amplifier and with an additional DC/DC power supply with an operational amplifier and bipolar transistors.
Figure 1.5: Simple circuit for high side sensing for positive power supplies with single differential amplifier without RIRO operational amplifier and with additional DC/DC power supply, operational amplifier and bipolar transistors.
IC2.1 works as a differential amplifier with a gain of ten. If needed, the secondary power supply V1 can be limited with a Zener diode up to Vz.
The switch S1shows two options for the power supply to the OA. The voltages V1 and Vz can be used also for other low power ICs in the equipment. We should note that both OAs IC1.1 and IC1.2 are parts from two different LM358 ICs.
Also we should mention explicitly that if we use OAs for current sensing in power supplies, we should add all necessary protection components in order not to damage the OAs in all cases of usage of the equipment. The protection components such as diodes, resistors and capacitors are not shown on the proposed figures.
Some current sensing circuits as the circuits in Figure 1.1 and Figure 1.2 may need additional buffering or amplifying in order to drive longer wires or low impedance signal condition circuits.
Figure 1.6 shows several simple examples for that buffering.
Figure 1.6: Simple circuit for buffering and amplifying the sensed voltage Va.
Figure 1.6a is a circuit from Figure 1.1 with an added emitter-follower with transistor T2 at the output;
Figure 1.6b is an ADC amplifier for Va with bipolar transistor T2;
Figure 1.6c is a buffer-follower for Va with an operational amplifier (OA) and protection of the input of the OA.
Figure 1.6a is in fact Figure 1.1 with an added emitter follwer with T2 at the output. This can be useful to drive the inputs of some of the ADCs.
Figure 1.6b shows a DC amplifier for the voltage Va with bipolar transistor T2. The gain of the stage is fixed with the resistors R5 and R6. C6 is preventing oscillation of the stage. We may use the operational amplifier to buffer, to amplify or to filter the signal from the current sensing transistor and that is the best solutions in most of the cases.
Figure 1.6c shows an example of a buffer-follower for Va with an operational amplifier (OA).
In this case we may need protection for the input of the OA with resistor and diodes and in some cases a voltage divider to reduce the input voltage within the allowed range of the OA.
Conclusions
The Part 1 of this short handbook, we presented simple, low cost and efficient circuits for high side sensing in positive power supplies. The circuits are built around PNP transistors and operational amplifiers. The circuits can be adapted to a large variety of applications, especially to higher voltages.
Some of the abbreviations in the text:
ADC – analog to digital converter
AC – alternating 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
D, VD – diode
R – resistor
RIRO – rail to rail input and rail to rail output
Vc – voltage proportional to the sensed current
PR – positive regulator
NR – negative regulator
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