PD 20V powered bench PSU  - Page 1
Forum section: electronics / projects

author: hbozyq
author_rank: Contributor
timestamp: October 13, 2024, 03:13:34 pm
content: Another portable bench power supply project. The main idea is it should be powered by 20V comes from 65W(or more power) PD charger and light enough to fit in my pocket.Target Features:DC 20-24V input0-20V 0-6A output rangeSMPS buck pre-regulatorsolid analog control loopadjustable over voltage clamp ( sinking power tolerate, useful while powering ESC and motors )CC short circuit tolerate (0V output stable)Programable (MCU needed)For now some feature are already tested, including pre-regulator and over voltage clamp. Here's my first design. Pre-regulator controlled by hysteresis comparator, Vpreg will always be regulated into a fixed amount (2v, with 600mV ripple) above Vout. The switching frequency is around 200-1.5KHz depends on output current. And 100% duty cycle can be achieved due to use of PMOS. Adjustable over voltage clamp is controlled by another hysteresis comparator, whenever the output voltage is A little more than setting, a brake resistor will be cut in and rapidly reduce voltage. Regen current had already blew several of my psu, and it's time to end it!The voltage control loop works fine ,but CC mode oscillate like hell, I think it might be due to low common-mode rejection ratio of INA180.

author: hbozyq
author_rank: Contributor
timestamp: October 13, 2024, 03:18:04 pm
content: New post-regulator, simulating seems fine except weird current spike, but I wonder if it works in field and what can be wrong.

author: hbozyq
author_rank: Contributor
timestamp: October 21, 2024, 08:06:48 am
content: INA180 oscillate a lot even in low-side sensing. Replacing it with a lower bandwidth device provide a better result (INA180A1 20x 350khz, TP199a1 50x 48khz).Or I have to struggle with loop compresation. Sadly I'm not familiar with that yet.Any way the cv loop works just OK, but I still need to figure out how to make it stable even without electrolytic capacitor.Another problem is the pre-regulator draw too much current while charging bulk capacitor, I got around 12a peak current (with low duty cycle), that's OK for transformers, but not for a poor tiny PD charger.Gonna to try some real smps controllers.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 21, 2024, 08:39:23 am
content: Some current spike is un unavoidable, from the capacitors at the output. There is no good way to get rid of the output capacitance, but one can get away with less capacitance (e.g. 10 µF electrolytic or with some ESR and some 100 nF low ESR (ceramic)). The ESR is a very helpful part for the stability.Another part of the current spike is from the time it takes for the CC loop to come out of saturation. This part can be made faster, but a little delay will remain.The circuit as shown may have a problem turning off the power FET fast - so load transitions from a high to low current may be an issue too. This would get more important with a smaller output capacitor.For the CV loop one usually will need an added resistor in series to C1 at the compensation. This gives another point for tuning the compensation (e.g. act as the P element in a PID type regulator).The difficult part for tuning the compensation is to make sure that the circuit does not oscillate with any reasonable load. Usually some low loss capacitance in combination with a constant current sink is the critical case for the CV loop. A helpful way is to simulate the output impedance. Oscillation can happen with a certain capacitance if the phase shift exceeds 90 deg. One may allow this for rather low frequencies (would limit the maximum permissible capacitive load). Getting too close to 90 deg (acts like a high Q inductor) would also cause more ringing.It is kind of impossible to avoid ringing all together for the difficult loads, but one should avoid oscillation over a rather large range (e.g. up to some 100 mF).

author: xavier60
author_rank: Super Contributor
timestamp: October 21, 2024, 08:47:08 am
content: If the CV loop becomes unstable, try parts of this design,https://www.eevblog.com/forum/beginners/lm324-power-supply-with-variable-voltage-and-current/msg3582664/#msg3582664Also, it's best to have the CV and CC opamps control the output in a separate way via ORing diodes.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 21, 2024, 12:00:10 pm
content: The diode OR (or better diode min value) is the most comonly used configuration, but this does not mean it is the best. There are actually also positive sides to the version where the CC loop lowers the set point for the CV loop. The shared integrator part avoids integrator run-up. The implementation may still want changes (e.g. less delay in the CC loop, maybe a nonlinear response for the CC loops to alow a faster off).

author: xavier60
author_rank: Super Contributor
timestamp: October 21, 2024, 12:20:57 pm
content: The speed of the TLC072 opamp and D5 on pin 2 in my design limits the short circuit current spike to a few us.Another smaller PSU I built uses a P-ch MOSFET for the positive rail pass element with no stability problems after the popper compensation was set. I'm expecting that Q1 in my design should be able to drive a MOSFET with no changes except for adding a Gate protection Zener.