Thursday, March 5, 2015

Boost Converter Output Capacitor Selection

I was a day late with this post as I was trying to select capacitors for my boost converter.  I knew it wouldn't be easy, but I didn't think It would be this hard.  The loose specification I'm trying to achieve is listed in this post, but basically I'm trying to convert 12 to 60 Volts driving a 300 mA load.

I started by looking at different converter topologies, and decided to go with a boost converter due to what parts I could get (I didn't want to hand wind transformers - walk before I can run).  I'm reasonably sure it's not the optimum topology, but it should be doable with off the shelf parts.  I'm at the point where I've got a stable design that does what I want, and am now trawling through suppliers to find actual parts.  Things were going great,  got an inductor, got a switching Mosfet, got a Schottky diode, hit a wall when it came to the output capacitor.

The problem is that the capacitor needs to be rated for 60 Volts, and to minimise ripple needs to be low ESR, and has to be around 5 uF.  That's like trying to find pixie dust.  Let's look at the equations that determine what capacitor I'm looking for and see if we can't tweak some things, keeping in mind that the purpose of the output capacitor is to minimise ripple in the output voltage.

Levels of Output Voltage Ripple in a Boost Converter

The output voltage ripple consists of two parts, the first part occurs when the capacitor discharges while suppling the load while the Mosfet is on and charging the inductor.  This means it's related to the duty cycle, frequency of operation, load current and the capacitor size.  I can't change the load current or the duty cycle, but I did decide to up the frequency of operation from 400 kHz to 600 kHz.  This means I can get away with a smaller capacitor.

The second part of the voltage ripple is caused by the ESR of the capacitor. When current flows into and out of the capacitor through the ESR during its charge discharge cycle, a ripple is created.  Reducing this ripple can be done in two ways, decrease the ripple current in the inductor, or get a low ESR capacitor.  I've increased the inductor size to reduce it's current ripple, but the best option is to select a low ESR capacitor.  Ceramic capacitors have such a low ESR they are essentially considered to have none, and for this reason have become a favourite in DC-DC converter designs.  When going through all my searches and considering specs against cost, they're pretyy much the best option.  They have one drawback though, when biased with a DC voltage their capacitance is reduced.  In some cases, significantly.  I'll let EEVblog explain.

So finding the capacitance I need isn't easy, particularly when I need to find parts rated to 60 Volts.  In the end I decided to go with 4 x 12101C225KAZ2A from AVX.  They're X7R 2.2 uF capacitors rated to 100 V each and when biased to 60 V I should get at least 550nF out of them, giving a total of 2.2 uF.

I need to investigate how the dynamic resistance of the load interacts with all of this, but in the end I think everything should work.  I've upped the frequency of operation, and slightly compromised on my acceptable level of ripple.  After all, isn't engineering is the art of intelligent compromise?

Electrical Schematic
Boost Converter Design


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