SMPS with 0.01% ripple
SMPS with 0.01% ripple
(OP)
I am trying to design a 10KV (Vout) SMPS with 1V peak to peak ripple on the output voltage. (No linear regulators are allowed here).
The load is a pulsed load and could be anything up to 250Kohms. The load duty cycle is 10% or less. And the Pulse repitition frequency is 200Hz to some 30KHz.
The SMPS will have a switching frequency of around 15KHz and will be of the 4 transistor bridge LLC resonant converter type.
First of all, does anybody believe that this low ripple is possible (i.e. 0.01% ?)
Also, if not possible, what do you believe is the lowest amount of ripple that can be achieved?
Also, do you believe that analog PWM IC’s will be best, or will one of the new programmable (DSP) PWM controllers with on-chip ADC’s be better for this kind of regulation?
(I know of one such IC which has a 500ns ADC read time (2MSPS).)
The load is a pulsed load and could be anything up to 250Kohms. The load duty cycle is 10% or less. And the Pulse repitition frequency is 200Hz to some 30KHz.
The SMPS will have a switching frequency of around 15KHz and will be of the 4 transistor bridge LLC resonant converter type.
First of all, does anybody believe that this low ripple is possible (i.e. 0.01% ?)
Also, if not possible, what do you believe is the lowest amount of ripple that can be achieved?
Also, do you believe that analog PWM IC’s will be best, or will one of the new programmable (DSP) PWM controllers with on-chip ADC’s be better for this kind of regulation?
(I know of one such IC which has a 500ns ADC read time (2MSPS).)





RE: SMPS with 0.01% ripple
I'll put my money on 1%.
Remember a switcher needs ripple.. That's what it controls with. No ripple.. Bad things happen.
You might as well give the dsPIC a try. The one for switchers. You should probably get the dev board and start hacking.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: SMPS with 0.01% ripple
I agree if it's a self-oscillating type, but who'd design such a thing today with all the excellent control chips available.
Anyway, 0.01% is far off the mark, I would even hope to be able to do that with a linear regulator.
As it seems pulsating loads are involved, the transient response issues wouls be enormous. I'm not certain whether the OP also calls this "ripple".
Benta.
RE: SMPS with 0.01% ripple
Thanks for these replies.
I am including the transient response situation in the ripple. The load is switching and it's not possible to tell exactly what the load will be when it switches on, the SMPS will just have to regulate well and keep the ripple down to 0.01%.
Apologies...in my original post, -i meant that 250K was Maximum Load.
I tend to agree that switchers need ripple. In fact an open loop SMPS with a fixed load would even have ripple, due to the fact that SMPS's kind of pulse energy to the output. In fact, this ripple seen in such an open-loop case would be the best case ripple achievable with that SMPS, if it later became regulated with variable load.
RE: SMPS with 0.01% ripple
There is always going to be some conflict between stability and response of the switcher, and a massively over filtered output.
My approach would be a very large output capacitor with suitably low Esr. If it is a pulse rated capacitor as used for high energy discharge applications, it will, by design, have a very low series resistance and inductance.
To achieve suitably low output ripple it will probably require some fairly significant series impedance between the switching power supply, and this output reservoir capacitor.
The trick is how to best arrange dc feedback to control the switching supply, so that it regulates reasonably well, and remains stable.
I have had success taking direct dc feedback right at the load point, but ac coupling the feedback direct from the rectified dc output from the switching supply. This ac coupling bypasses any inconvenient phase lag in the output filter components, but dc feedback is still sensed right at the load.
I am sure with a bit of fiddling this will work for you, it may even require two smaller stages of filtering at the output. A lot depends on the exact nature of the load variations.
RE: SMPS with 0.01% ripple
Power it with an isolation transformer, give it a -15 to +15 V output range controlled by an optocoupler, and put it in series with the switcher output.
And give it really good transient protection.
RE: SMPS with 0.01% ripple
RE: SMPS with 0.01% ripple
RE: SMPS with 0.01% ripple
RE: SMPS with 0.01% ripple
"use a high frequency switching frequency so the loop can respond quickly. "
Regarding the above, kindly received from Logbook, I believe you are right, -however, there is some thought now that we want a low SMPS transistor switching frequency....this is to reduce switching losses and keep the switching transistors as cool as we possibly can.
But i can see that this low frequency switching is entirely in opposition to our wish to have low output ripple. So this is a sticking point.
RE: SMPS with 0.01% ripple
For example, use four phases at 15 kHz, and stagger them so they switch ~67 microseconds apart (90 degrees of phase). That feeds power pulses to the load at 60 kHz.
RE: SMPS with 0.01% ripple
Primary ripple rejection can be made arbitrarily low by filtering the input and using a current mode controller. The resulting increase in cost and reduced efficiency has to be considered.
Secondary load rejection is improved by
a) faster loop response
b) secondary regulator
c) feedforward load compensation
We have discussed a and b so far.
If you know a load is about to switch you can "unswitch" another load or boost the primary power. This is an open loop correction but if adjusted suitably will probably reduce the ripple by a factor of at least 5x. If the load is arbitrary and uncontrollable then this method will not work.
RE: SMPS with 0.01% ripple
"Analog PWM is the way to go. DSP is going to yeild too much bit error"
With reference to this, with dsPIC30F2020, i can get 10 bit resolution for feedback voltage with the on-chip ADC. Also, the sample rate is 2MSPS.....do you think this is going to badly affect my output ripple? (i.e. compared to analog PWM)
Also, with reference to the 0.01% ripple spec for output voltage.......would any reader know if this is possible using just one SMPS stage with "no extra bells & whistles"?...eg, no post-regulator, no extra filter.....just the basic topology with its secondary rectifier and LC filter (assuming its a 4 transistor bridge, say).
RE: SMPS with 0.01% ripple
RE: SMPS with 0.01% ripple
Dan - Owner

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