I'm building buck converter to drive led lamps. It consists of microcontroller, PWM generator, buck-circuit and current sensor. Microcontroller knows what target current should be and sets duty cycle basing on current sensor results.
The circuit, powered with +48V at VIN should drive led lamps with wide voltage and current ratings. Current limit is set in uC, independently for each lamp. Output must be filtered for as-low-as-possible ripple, because lamps are pretty far from the driver and I want to reduce EMI.
First simulation: 30kHZ PWM Frequency, 50% duty cycle, 1mH inductor, 47uF capacitor. With this circuit I want to drive 2.5A led strip.
However, in simulation, I can see very high overshoot current during first 2ms of operation. After that current is pretty stable.
I can't just put passive limiter there, because I have to retain ability to set output current.
Now, my questions:
- Will such current raise (3.6A for 2ms) destroy 2.5A rated led strip?
- How can I prevent such effect?
- What can I do to achieve better filtering of output current?
- Is there any way to reduce inductor size? With 100uH I get lots of ringing on drain
- How can I improve the circuit?
Best Answer
The problem is in your control loop. This is the firmware that senses what the supply is doing, compares that to what you want it doing, then adjusts the PWM duty cycle as a result.
You haven't said anything about your control loop, so there is little we can say about it either. In general, it is too aggressive. In your situation it seems that slow response to a new situation isn't a big deal. LEDs are pretty fixed in their characteristics. You can therefore overdamp the controller. That will greatly reduce or eliminate overshoots, at the expense of slow transient response.
There are whole books on how to optimize control loops. However, fancy control schemes are often not needed. I'd use a simple pulse on demand system. When the output falls below the regulation threshold, you do a pulse, else you don't. This has more ripple, but is nicely stable with no more overshoot than what a single pulse can produce.
This is exactly the control scheme I used in my KnurdLight LED headlamp. The control scheme is so simple it runs in a PIC 10F.