Switch Mode Power Supplies (Part 3)

Part 1, 2, 3

Current sensing. This part is probably the most critical and most uneasy. It is critical because current sensing will protect both the power supply and the driven load from excessive currents and thus excessive amounts of energy dissipated in both the source and the receiver. There are multiple designs available for current sensing. I found interesting to review some of them.

Let’s start with the most unusual, although popular in the early 80’s electronics: the coil sensor. The output current flows through a coil which surrounds a reed switch.

BTW, this coil is from an HP5840 A25 board. If you ever worked on this product, and survived the risky business of troubleshooting this board, please send me an email ūüėČ

If the current rises, so does the magnetic flux. If the current exceeds a certain level, the magnetic flux is high enough to close the reed switch. This switch is connected to the power supply driver and triggers an – almost – immediate stop. The major advantage of this design lies in its ability to withstand heavy currents which have almost no effect on the voltage drop across it. and consequently this circuit dissipates very little energy.

The most popular sensing device is the shunt resistor. According to the Ohm law, the more the current flowing through a resistor the more the voltage drop across the resistor. All we have to do is to insert a resistor within the receiver loop and measure the voltage drop across this resistor. There are mainly two type of designs: the resistor is attached to the low side of the circuit and the resistor is attached to the high side of the circuit. In the early days of electronics, both following designs were very popular: both feature a transistor which base emitter junction is biased by the voltage drop across the shunt resistor.

Shunt resistor on the low side:

As soon as the voltage across R2 exceeds 0.6 V, T1 is turned fully on and switches the feedback line to the regulator to ground. R1 acts as a current limiter.

Shunt resistor on the high side:

As soon as the voltage across R2 exceeds 0.6 V, T1 is turned fully on and switches the feedback line to the regulator to Vout. R1 acts as a current limiter to the base of T1.

Along with the popularization of ICs, Op. Amps replaced the transistors allowing lower resistor values (and thus lower power dissipation) as well as more accurate control.

Shunt resistor on the low side

In this case we use an op. amp. configured as a non inverting and amplifying stage. This is a simple design which suffers from multiple draw backs: the load has no connection to ground, voltage regulation is made uneasy and the input will pickup all the ground disturbances.

Shunt resistor on the high side

In this case we use a differential amplifier. Although this design looks nice and easy, it requires very particular attention. Using resistors featuring standard tolerances will generate errors and the op. amp. must cope with relatively large common-mode signals. In addition, none of the load terminals is attached to ground making shorts to ground detection impossible.

In both cases, the op. amps must feature a rail to rail outputs.

Chips makers designed specific, small, low cost ICs containing the amplifying stages and their ancillaries. In this way the design of the current sensing stage is easy with a low foot print and a tidy design. In some cases an appropriate design of the copper layer will be substituted to the shunt resistor. Next is an example among many, featuring the TI INA193 IC which is available in sot_23_5 packages and various gains (x20, x50 and x100)

Some other designs get rid of the resistor and will directly measure the current flowing through the circuit. These chips feature a hall sensor which is orthogonal to the current flow. The major advantage of this design is the very low voltage drop across the sensing element; consequently, they are very well suited for high current sensing, low foot print and little heat dissipation. In addition they offer an excellent galvanic isolation and they are very suited for measuring current flows in both directions. Check this component which is also available on modules for fast prototyping : Allegro ACS723

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