1.5amp 12v DC to DC converter design from 32v supply

Thread Starter

waylander_jobby

Joined Aug 20, 2020
1
Hi guys, ive been hunting around and have found an off the shelf PCB item that does this however when loaded up to 1.5amps it seems to sag to 6v instead of 12, and the whole design using 3 different boards is ugly and i want to move away from this. I am using this to supply a flashing cree led light from a 32volt headlight supply. Cree leds take 12volts.

What i was looking to do instead is design my own switching DC to DC converter to drop the nominal 32 volt input (generally runs at 32v but needs to be able to handle a transient peak up to 50v max, source power supply is a steam turbine that has a slowish acting mechanical governor, this cant be changed). The maximum supply current needs to be minimum 1.5 amps, 2 amps is more ideal if possible and output voltage nominally 12v (a tiny little bit over or under wont affect things too much), however when the flasher isnt operating the current draw will drop to 20 to 30mA but the output still needs to remain at 12v.

I just cant get my head around the design for a switching DC to DC converter.
and a Linear voltage regulator will get too hot dropping that power.

can anybody help me please.
 

MrAl

Joined Jun 17, 2014
8,231
Hi guys, ive been hunting around and have found an off the shelf PCB item that does this however when loaded up to 1.5amps it seems to sag to 6v instead of 12, and the whole design using 3 different boards is ugly and i want to move away from this. I am using this to supply a flashing cree led light from a 32volt headlight supply. Cree leds take 12volts.

What i was looking to do instead is design my own switching DC to DC converter to drop the nominal 32 volt input (generally runs at 32v but needs to be able to handle a transient peak up to 50v max, source power supply is a steam turbine that has a slowish acting mechanical governor, this cant be changed). The maximum supply current needs to be minimum 1.5 amps, 2 amps is more ideal if possible and output voltage nominally 12v (a tiny little bit over or under wont affect things too much), however when the flasher isnt operating the current draw will drop to 20 to 30mA but the output still needs to remain at 12v.

I just cant get my head around the design for a switching DC to DC converter.
and a Linear voltage regulator will get too hot dropping that power.

can anybody help me please.
Hello,

Before you design or buy, either way the first thing you should do is investigate the reason the previous one drops to 6v. It could be that the regulator is not good enough, or it could be that the input power isnt good enough. If it is the regulator then a new regulator would solve the problem, but if it is the input source then no regulator alone will solve the problem. I will explain.

If for example you are getting 12v at 1 amp with no problem for a while that means you have enough input power which would have to be at least 12 watts (but really a little higher than that). Now if the voltage drops to 6v with the old regulator the input power could have dropped to less than 12 watts (like 9 watts for example) and that is why the output can no longer support 12v at 1 amp.
If this is the case, then you have to increase the input power and then later decide if the regulator is good or not. If the output only has to supply power once in a while, then you might get a way with a battery charging system but for continuous operation you need to increase the input power which may mean you just have to be able to generate more energy first.

To find out what case is causing the output to fall, measure the input voltage and if possible the input current. If the voltage is still high enough (something like 13v or higher) then it may be ok and a new regulator will help, but if the voltage falls to 11v or 10v or 9v or lower then you need a better generator or a second generator.

If you just get a new regulator and that doesnt help either, then you also know the input is probably not getting enough power but a simple test is to measure the input voltage when the output drops and see if it is high enough.

Be aware that if the input is 12v then you need at least 1 amp in, but if it drops to 6v then you need at least 2 amps input because 6*2=12 watts. If the input drops to 8v then you need at least 1.5 amps in because 8*1.5 also equals 12 watts. If you get lucky and the input goes up to 24v then you only ned 1/2 amp in because 24*(1/2) also equals 12 wstts.
I am using numbers for a regulator that is 100 percent efficient for simplicity, in real like you need a little bit more power and you should really figure at least 20 percent more input power than output power. That means for 12 watts out you really should make sure you have at least 12*1.2 which is 14.4 watts input power available at all times.
 
Last edited:

RPLaJeunesse

Joined Jul 29, 2018
164
If you think the peak input is 50V max then plan on it being higher than that for short periods due to inductive kicks that can happen. I'd use a heavy 40V transient supressor (65V max!) and a 65-75V rated buck step-down stage. An LMR36520 would survive that as a 2A rated buck regulator, or do a design with MIC2176 and external FETs good for 3-5A. There is an online "Webench" design tool for the LMR36520 part to tell you all the part numbers.
 
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