How to make this high side current limiter more predictable [closed]

Question

I started out experimenting with a very basic current limiter that begins to pinch off a pmos when about 0.7 V drops across a shunt

I then decided I wanted to be able to trim the current with a pot and have a bit more predictability, so moved to a design with a dedicated current sense amplifier. The following circuit basically functions on the breadboard, but the current limit is not a hard threshold. It varies with R1 and R3. The current sense amplifier is a MAx4080S with a gain of 60. I'm doing a bit of a hack where I use a ground reference off a zener, so when theres no current across the shunt, the output is zero (i.e. about -10 V) and as current flows through the shunt, the voltage drops and is amplified and pinches off the pmos. I'd like it to work more predictably if the source voltage sags or the load impedance changes. How could I go about improving this design?

Additional info: This circuit operates off a PWM drive signal (sometimes 10% driven in the range of a few kHz) from another circuit that is low-side switched. The regulation needs to kick in fast. The 48 V might sag down to 40 and the load impedance is unstable. I need to drive everything off the 48 V(ish) supply I have (hence choice of 4080)

A scope across the load shows current oscillating at 83 kHz! Why (there is a 100 Ω resistor between the output of the amp and the gate)? Schematic 3 below removes the oscillations but the integrator wind up is huge (~500 µs)

Answer 1

The regulation needs to kick in fast. The 48 V might sag down to 40 and the load impedance is unstable

Q1 and Q2 should be a dual matched transistor like BCM857. These dual transistors have pretty close Vbe match if both run at the same operating point and at the same temperature. Q2 is wired as a diode with a constant current bias (I2) and creates a voltage drop that cancels Q1's Vbe. This makes a current limiter by pulling on the MOSFET gate, with the current limit set to zero instead of one Vbe across R1. Current mirror Q3,Q4 applies a current across R4 to add some threshold voltage back, which sets the current limit. Thus the control input is I3.

This has a bit higher offset and drift than an opamp, so it will be less accurate, but it is faster because the bandwidth of the error amp (Q1) is higher than what the usual current sense opamp offers. The MOSFET gate is current driven and compensation is provided by the MOSFET's gate capacitance and Miller effect.

I added a low side switch and load resistor. The current limiter acts in about 170ns:

To suppress this overshoot, a solution would be to re-architect the circuit: instead of a low side switch and a high side current limiter that acts "after the fact", you could use a low side switched current source. Instead of overshoot, this will result in a delay to ramp up the current, which can be a better option.

On second thought, the current mirror resistors are too high at 1k, a lower value is better to avoid saturating Q4.

Answer 2

Perhaps this might help. I have a working high current 12-volt power supply. I have used the design in roughly 50 DC power supplies with voltage and current limiters replaced with this LDO regulator. It looks a little busy initially, but isolating the circuits is straightforward. The current limiter consists of R1, R2, R4, and U3. The output of U3 is diode OR’ed with U2, the voltage regulator, to control the output of M1.

The design allows for exact voltage and current control using a precision voltage reference created by U1, R7, C1, and C2. The output of U1 provides the reference for both the Current and Voltage regulators.

To control the current, replace R2 with an appropriate-sized resistor or potentiometer. For example, 100 to 500 Ohms will produce 5 to 30 amps output. Ignore R5, which is a Voltage Foldback resistor.

If you eliminate U2, D1, R12, and R13 (the voltage regulator), you have a precision current supply. The Op Amp must have a high CMRR and PSRR to avoid the high-side current limit/monitoring.

Hope that helps

Answer 3

You need an op-amp and reference voltage, not a current amplifier. The amplifier is fixed-gain, which sets output current in terms of Vgs(th), which is poorly defined. In both cases, you likely need compensation to avoid oscillation, as the MOSFET has high capacitance

^{simulate this circuit – Schematic created using CircuitLab}

Note you'll still have "integrator windup", i.e. normally while the output is saturated, the amp is saturated to +V, and it will take some time to slew up to Vgs(th) and begin regulating. This manifests as a "diode reverse recovery" sort of response, the current peaking initially before settling down, the peak amplitude varying with rate (dI/dt):

With proper tuning (choice of op-amp, MOSFET and compensation components C1, R4, R7) this can be reduced to a few µs easily enough.

Answer 4

Your first circuit doesn't work the way you want because you lack a precision reference - Vbe is not a precision voltage reference. Your second circuit has the same problem as the first, but this time, the amplifier is unstable because you have no compensation for the capacitive load presented by the MOSFET gate. Here's how I would do the circuit that will do what you want:

^{simulate this circuit – Schematic created using CircuitLab}

This is a High-current, constant-current source. But it will function as a current limiter.

There are simpler solutions available if you are doing low-side sensing. Even simpler if you're load was only a few hundred mA. The issue is detecting a lot of current while minimizing heat in the shunt resistor. INA293 and R1 could be replaced with a hall-effect current sensor. The exact details of the design will depend on your application requirements.

Edit: updated schematic per @TimWilliams observation.

Per comments, here's the simpler low-side version:

^{simulate this circuit}

Or with the Darlington:

^{simulate this circuit}

This one also has an added unregulated power supply to bring down the 48V. It could be used in all three circuits I posted. There's a lot of flexibility here so the parts aren't critical and performance can be easily tailored.