High power synchronous boost converter.

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Ev8
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High power synchronous boost converter.

Post by Ev8 »

Ok I may well need talking out of this one! But hear me out, before I relent and increase my battery voltage I’d like to be able to test some higher voltages,
I have 3 inverters worth of Prius gen2 boost inductors and ipm’s sat here doing nothing, I found some evidence to show the inductors are good for 150a before saturation, so with my current 80s pack I could potentially build a standalone synchronous converter capable of >100kw ish
Now I get that I’ll need to do some load banacing which should be possible using current sensors for feedback and simple pi control,

My main concern is that if all converter outputs are tied to a single dc bus output (the inverter) without some form of output diode am I just paralleling the component values rather than paralleling 3 separate circuits, for example paralleling the inductors will lower the overall inductance while paralleling the capacitors will increase the overall capacitance,

I guess asking this question probably means I shouldn’t even think about it but I’m that guy
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Re: High power synchronous boost converter.

Post by johu »

When fully independent with current control each it should certainly work. Plus I want to see that done :)
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Re: High power synchronous boost converter.

Post by Ev8 »

Ok I’m going to give it a go, watch this space
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Re: High power synchronous boost converter.

Post by Pete9008 »

This sounds interesting. Looking forward to seeing this working!

One word of caution, no problem with a simple PI control loop but make sure it is fast too. Say you are running at high load and suddenly come off the throttle - if the boost converter doesn't back off fast enough it will over-volt the DC bus and fry the inverter! Not sure of the values for the Gen 2 so guessing a bit but say 80kW load, 300V pack so ~250A, assume 2000uF (??) DC bus capacitance and 100V headroom (400V bus with max of 500) you would have 800us (T = CxV/I ) for the boost converter to back fully off before it over-volts the inverter. This is worst case as an instant load drop is unlikely (unless the inverter trips out) but worth thinking about. If you add regen (intentional or non-intentional) things get worse.

On the load sharing I'd be inclined to wire the three up separately (3 sets of L, IPM and C) but still run them from a common PWM drive signal. This way you can just run a single DC bus voltage control feedback loop. If you go for individual control then you need a current sensor on each, you need a current feedback loop for the load sharing with a voltage feedback loop running on top for DC bus voltage. There is also the potential for fighting/oscillation due to interaction between the loops. Running them from the same PWM signal will still share to a degree, controlled by the tolerance of the inductors (which is probably pretty good if they are all from the same inverter family). As long as you allow enough margin before inductor saturation the sharing like this should be fine. It would also be possible to run at low load, use a clamp ammeter to check the load sharing and then tweak the individual PWMs to improve balance further (3pwm channels but with individual offsets to improve sharing and a common control loop) while keeping the single control loop.

Final thing to think about is whether to run it as a boost only module, and disable all regen, or run it as a buck/boost module with regen. Thinking about how the control loop would cope with the transition from buck to boost is a bit beyond me at the moment (it might just do it but not sure) so will let my subconscious mull it over for a while. On the Gen2 buck boost do you have individual control for the high and low side IGBTs or do they share a single line like the inverter ones?
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Re: High power synchronous boost converter.

Post by arber333 »

Ev8 wrote: Mon Jul 18, 2022 6:41 pm Ok I’m going to give it a go, watch this space
Hm... would a really small tough and powerfull 300Vdc battery inside your boost converter help any? it wouldnt have to be large in size and capacity, but it would have to be though in power. Maybe LiTO battery or some supercap bank? Then you could have a slow and uncomplicated boost converter with single variable to follow = voltage. You wouldnt need precise current measurement or you could just assume it from main DC bus current sensor.
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Re: High power synchronous boost converter.

Post by Ev8 »

Some good points to think about, pretty sure as a synchronous converter regen shouldn’t be a problem because it shouldn’t behave as a buck converter when current flows back through it the other way, at least thats my understanding
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Re: High power synchronous boost converter.

Post by Pete9008 »

Having pondered on it I think you're right and the buck/boost control will work fine. Still worth a bit of caution when trying it the first time though!

As long as you can get the control loop to be quick and stable enough it should work well, after all you are using the bits in exactly the way that they were designed to work.

I've been trying to figure out how to get the Prius Gen3 buck/boost module to work, with both forward boost and forward buck, as a charger using an additional input ips stage. That would have had some some nasty control loop discontinuities (especially as I wanted some degree of PFC too). Given up on that now though and bought a Volt charger instead. I'm still hoping to use the buck/boost module to provide an interface to a supplementary battery pack though so very interested in how you get on.
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