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1kW resonant SMPS

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4th March 2015, 08:13 PM

#1

mag

diyAudio Member

Join Date: Jan 2008

Location: Chiasso

1kW resonant SMPS

Hi All,
I have finished building a 1kW
SMPS with PFC that I designed to power up
my class AB audio
amplifier.
The main characteristics are:

Input voltage:
180...265VAC
Output voltage: +/-55VDC
Output current: 10A/rail
--> total output power 1.1kW
Topology: ZVS Series resonant half
bridge with BCM PFC. The switching
frequency is the tank resonant
frequency or slightly above it
to ensure ZVS of bridge FETs. The output
voltage is not regulated
and basically tracks the PFC voltage (385V)
which is loosely
regulated.
Resonant frequency: 100kHz
PFC
Voltage: 385VDC

Magnetic
parts
----------------

Transformer: core: ETD44 3F3 no gap
material
primary: 28T 100x0.1 Litz wire
secondary: 2x8T 200x0.1 Litz
wire

Resonant choke: core: PQ2625 3C90 material
inductance: 27uH
gap adjusted for proper L
winding: 18T 120x0.1 litz wire

PFC
choke: core: PQ3230 3C90 material
inductance: 100uH gap adjusted for
proper L
winding: 30T 120x0.1 litz wire

Note: this SMPS is
designed for peak power of more than 1kW, is not intended for continuous
use at 1kW (transformer too small....).

I have finished to build it
but not yet powered up.... I am a bit scary of what can happen if
something goes wrong....

Comments and questions are
welcome.

ciao

-marco

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4th March 2015, 08:25 PM

#2

rikkitikkitavi

diyAudio Member

Join Date: Dec 2001

Location: South Sweden

Since you already use two resonant caps, why
dont you use a split capacitor layout + 2 fast diodes antiparallell, then
you will have a shortcircuit proof design
( google for split cap llc ,
or look into the usual appnotes)

Did you add c17 and c27 to
stabilize coss to have a better control of nevessary deadtime?

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4th March 2015, 08:32 PM

#3

mag

diyAudio Member

Join Date: Jan 2008

Location: Chiasso

Hello rikkitikkitavi,
you are right, I did
not thought about it... but now the PCB is done.
The small 100pF caps
on the FETs are there but not mounted.
On the PFC FET they are used to
limit the dv/dt if needed on the bridge
are there just because there
was place but I don't think that 100pF really matters here.

mag

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5th March 2015, 12:02 PM

#4

rikkitikkitavi

diyAudio Member

Join Date: Dec 2001

Location: South Sweden

No probably yoou wont need the 100pF caps.
The internal capacitance of the fets is about 3-500 pF (or something) and
since you use CFD (fast recitifer tailored towards the resonant converter)
it will give you are more reliable converter

(there is plenty of
white papers from semiconductor manufacturers describing why it is a good
idea to use THEIR proprietary low Qrr,Fast Trr body diode mosfets for
resonant converters but in principle it is same same but different...)

Awaiting first power up.

Good luck and no smoke!

Ps - try to start the PFC first with a resisitve load...

Ds-But one thing I dont understand, without feedback how do you
control the resonant frequency ? Or did I missunderstand so that it is
constant frequency and constant duty cycle?

regards
Rickard

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5th March 2015, 02:39 PM

#5

mag

diyAudio Member

Join Date: Jan 2008

Location: Chiasso

It is an LC series converter not an LLC
converter.
The frequency is locked at fres or slightly higher to ensure
ZVS and the duty cycle is fixed at 50% (minus dead time).

The
frequency is set with a trimmer and there is no feedback loop.

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5th March 2015, 08:13 PM

#6

rikkitikkitavi

diyAudio Member

Join Date: Dec 2001

Location: South Sweden

But you have two caps (47nF) and a 27uH
inductor in series with the primary winding, hence C L L = LLC .

But the reflected primary inductance will change with load.

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5th March 2015, 08:17 PM

#7

rikkitikkitavi

diyAudio Member

Join Date: Dec 2001

Location: South Sweden

Right, some google fu gave this old app note.
Will read...
http://www.ti.com/lit/ml/slup085/slup085.pdf

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7th March 2015, 09:39 AM

#8

mag

diyAudio Member

Join Date: Jan 2008

Location: Chiasso

In fact it is always an llc but the
magnetizing inductance is much higher than the resonant inductance.
If
the frequency is set at f0 we are at the load independent point where the
output voltage does not change with load current (except losses). I don't
understand your point about primary inductance changing with load.
The
frquency is locked at f0 so that the output voltage will be vout=vpfc/n
since the gain of the llc tank is practically 1 at f0

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8th March 2015, 01:08 PM

#9

rikkitikkitavi

diyAudio Member

Join Date: Dec 2001

Location: South Sweden

ah, yes youhave two fr, one including the
magnetising inductance and the other not.

Sorry for beeing
unclear, with reflected inductance I meant all stray and leakage at
primary side and reflected from secondary side to primary, so in practice
your resonance frequency will be sligthly lower(I guess 27uH will be about
30-35uH). But you err on the high side, just dont loose your
zvs
..

How is your xformer wound?

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8th March 2015, 09:41 PM

#10

mag

diyAudio Member

Join Date: Jan 2008

Location: Chiasso

The transformer has around 2.5uh of leakage
inductance. It is wound in this way:
- bobbin
- secondary 1: 8 turns
200x0.1 litz wire
- primary: 28 turns 4x0.65 tex-e wires in
parallel
- secondary 2: 8 turns 200x0.1 litz wire
- aux winding: 3
turns 0.2mm tex-e wire

I have started testing it, the PFC works
fine, tested up to 600w with resistive load.
The llc has some problems
with the self supply; due to the big output capacitance the
output
Volrage and thus the auxiliary voltage rises slowly. The
auxiliary voltage being too low the smps continuosly starts and stops. Now
I have added another auxiliary winding on the resonant choke that will
supply the controller when the output voltage is low. Not yet tested it
but from simulation it seems to work fine.

Regading loosing zvs I
don't think it will be able to keep zvs at no load or very light load, the
magnetizing inductance is too high. Possibly the transformer can be gapped
to increase the reactive current keeping it always in zvs like a standard
llc converter. I will see if it worth to be done-

mag

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