buckdc/dc converter
Are there any limitations of connecting two non-isolated buck converters in series? Just taking the output from the first converter and connect it as the input of a second converter to produce two DC voltages.
I've seen and tried this before but are there any relations between the two switching frequencies that I should avoid or any other parameters?
I will try to answer your points assuming I have read them correctly.
1) The driver for the buck circuit will generally need some kind of regulation, but this can be a low power regulator. You then use the buck circuit to step down your supply voltage and regulate it, this will provide a lower stable voltage with more current for your load.
2) If your have either a dynamic supply voltage or load then the buck converter will need to be regulated to maintain a stable voltage. This is achieved by varying the duty cycle to the power MOSFET and feedback is required to the driver circuit to determine how the duty cycle is adjusted. You can use voltage feedback and then some closed loop control circuit to alter the duty cycle pending on the feedback signal.
To summarise a buck circuit is a good way to step down and regulate a large voltage to a smaller voltage, this can then provide power to a small, medium or large load. Additional regulation is generally required for control circuitry such as a microcontroller and driver circuit, but these circuits generally have very small power requirements.
Do you have an alternative approach I haven't thought about?
Find a generic boost converter that can reliably produce 17 volts at 600 mA then, rearrange the feedback loop with your LEDs as part of it: -
The picture above uses a 12.4 ohm resistor and a FB voltage of 250 mV in order to get 20 mA but there's little stopping you using a 1.24 ohm resistor and getting 100 mA.
If you pick a generic booster that has a 0.8 volt reference voltage for feedback and you want 600 mA to flow, the resistor would need to be 1.333 ohms.
Best Answer
Make sure you have ferrite beads that can filter out the higher harmonics of the 2nd buck switching frequency.
simulate this circuit – Schematic created using CircuitLab
This ferrite is shown by L1. Suppose that U2 switches at 200kHz, the MOSFET/switch, keeps switching at that frequency. There will be an impact on the Vin (P1) of the 2nd buck. That noise might percolate ionto the input rail of the 2nd buck or rather the output rail of the 1st buck. As such you might not have much issues. But are you providing the output of the 1st buck to other loads (sensitive) ? if so,especially in case of RF load, this noise can be a killer.
Filter out this with proper ferrites and capacitor banks.