Input Filter Magic

Webinar February 23, 2023

Dr. Ray Ridley

© 2023 www.ridleyengineering.com 1
 Input Filter Modeling – the Players

Dr. Shriram S. Kelkar broke new

Dr. David Middlebrook presented ground by trying to control the
the problem of the input filter effect of the input filter. He came
interaction with a power supply as close to getting it right. The
part of a bigger general important thing was the concept
impedance theorem. that you can control things
outside of the converter cell.

Dr. Vatché Vorpérian – the best

Dr. Vlatko Vlatkovic found the analytical mind in power
issue with the feedforward electronics. Without his PWM
solution to input filter switch model, none of this work
interactions. would have happened.

Arthur Nace –aerospace engineer

and programmer who automated
LTspice models for us and made
this work possible. Our longest
user of RidleyWorks.

© 2023 2
 Part I

Basics of Input Filter Measurement and Design

© 2023 3
 Middlebrooks Input Filter Problem

Middlebrook’s original problem description

Practical considerations for system testing

InputFilterMiddlebrook1976
Necessary measurements

© 2023 4
 Middlebrooks Input Filter Problem

InputFilterMiddlebrook1976

© 2023 5
 Making Impedance Measurements

© 2023 6
 Making Input Impedance Measurements

This is the most difficult of all transfer function measurements to make.

If you can avoid it – then avoid it!

Even if you measure it successfully, it’s probably wrong anyway!

© 2023 7
 Making Impedance Measurements

© 2023 8
 Making Filter Output Impedance Measurements

This measurement is easy. No need to power the input filter

Make sure you short the input of the filter for the measurement

© 2023 9
 Making Filter Output Impedance Measurements

2
-V
P

This measurement is easy. No need to power the input filter

Make sure you short the input of the filter for the measurement.

© 2023 10
 Making Filter Output Impedance Measurements

1. Identify the components of the input filter.

2. Identify the components of the switching power supply. Do NOT include filter components.

3. Place a short circuit on the input to the filter.

4. Measure unpowered filter output impedance with a device such at the AP310 or RidleyBox.

5. Plot the CALCULATED input impedance of the power converter on the same graph.

6. Ensure good separation between the two curves.

7. Check that the filter is well damped to prevent ringing.

© 2023 11
 Part II

Going Deeper – Models and Loops

© 2023 12
 LTspice Model to Simulate Input Impedance Measurement

Estimate the input impedance as V^2/P. Use low line for lowest value

Circuit automatically generated from RidleyWorks

© 2023 13
 Calculating a Fixed Input Impedance is a Conservative Approach
29.54 dB

2
Estimate the input impedance -V . Use low line for lowest value
P
Actual impedance will rise above the crossover frequency.

© 2023 14
 Going Deeper – Interaction with the OPEN-LOOP Input Impedance

Z out
Z in

Middlebrook talked about avoiding the open-loop input impedance of the power supply

This will mean that the transfer functions of the original power stage won’t change significantly

© 2023 15
 Middlebrook’s Classic Filter Interaction Loop Gains

01 AVERAGE SMALL SIGNAL VM Filter

© 2023 16
 Input Filter Damping

Rc = 3 mΩ

Rc = 30 mΩ

Rc = 200 mΩ

© 2023 17
 Filter Interaction Loop Gain

No Input Filter

Rc = 200 mΩ

© 2023 01 AVERAGE SMALL SIGNAL VM Filter 18

 Filter Interaction Voltage-Mode Loop Gain

Rc = 30 mΩ

Stable – High Q LHP zeros

Rc = 3 mΩ

Unstable – High Q RHP zeros

© 2023 01 AVERAGE SMALL SIGNAL VM Filter 19

 Unstable System Simulation

02 TRANSIENT VM Filter

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 Part III

Input Filter FeedForward (Feedback?)

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 Input Voltage Feedforward

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 Input Voltage Feedforward

Vin

Ramp slope is determined by the input voltage

Input voltage goes up – duty cycle immediately goes down. (Feedforward)

© 2023 23
 Input Voltage Feedforward

Feedforward
-kf

01 AVERAGE SMALL SIGNAL VM Filter

As the input voltage increases, the duty cycle decreases.

© 2023 24
 Effect of Input Voltage Feedforward

Rc = 3 mΩ

3 mΩ with feedforward

© 2023 25
 Current-Mode Loop Gain with Undamped Filter

03 AVERAGE SMALL SIGNAL CM Filter

Like FF voltage-mode, the loop is unaffected by the filter!

© 2023 26
 Transient Simulation with Undamped Filter

04 TRANSIENT CM Filter

Output voltage looks stable!

© 2023 27
 Transient Simulation with Undamped Filter

04 TRANSIENT CM Filter

Beyond 5.5 ms, the converter is clearly unstable.

Phase margin in loop is 60 degrees. But the system oscillates!

© 2023 28
 Transient Simulation with Undamped Filter

Filter Output Voltage

Converter Output Voltage

04 TRANSIENT CM Filter

The system is always oscillating – you just can’t see it at first

© 2023 29
 Unstable and Unobservable System

Very high rejection of input voltage due to feedforward

Oscillations on the filter cannot be seen on the output

© 2023 30
 Unstable Loop Gain Before Feedforward

What’s going on here?

01 AVERAGE SMALL SIGNAL VM Filter

© 2023 31
 Unstable Loop Gain Before Feedforward

180 degrees phase drop

Two RHP zeros

Feedforward eliminates the two RHP zeros completely. How?

© 2023 32
 Feedforward is Actually Feedback!

Feedforward
-kf

01 AVERAGE SMALL SIGNAL VM Filter

Gain from a source to duty cycle is feedforward

Gain from a state to duty cycle is feedback

© 2023 33
 Feedforward is Actually Feedback!

Feedforward will move the zeros of a transfer function

Feedback will move the poles of a transfer function

“I’ve seen this idea presented for the last three

years at this conference. Something about it
really bothers me, but I can’t quite put my
finger on it.” Middlebrook c.1984
© 2023 34
 RHP Poles in the Transfer Function
Feedback will move the poles of a transfer function

Feedback can cancel two RHP zeros with two RHP poles right on top of them!

01 AVERAGE SMALL SIGNAL VM Filter

RHP poles in the transfer function!

© 2023 Nyquist must encircle the -1 point as many times as there are RHP poles 35
 Part IV

Design the Feedback System Properly

© 2023 36
 Input Voltage Feedback

05 TRANSIENT Filter Magic E

Notice the polarity – Input voltage goes UP and error signal goes UP

Opposite to feedforward technique from Kelkar and others.

Everybody missed this!

© 2023 37
 Turning the Magic ON

Input Filter Voltage

Filter Feedback On

05 TRANSIENT Filter Magic E

Filter feedback circuit perfectly controls the oscillation

© 2023 38
 Output Voltage Startup Simulation

Filter with Feedback

No Feedback

05 TRANSIENT Filter Magic E

© 2023 39
 Input Voltage Feedback Gain

06 TRANSIENT Filter Magic E GainSweep

Optimize the gain through simulation (with knowledge of the Bode plots)

The compensation shape can also be experimented with or analyzed. (7th order system)

© 2023 40
 Input Voltage Feedback Gain Optimization

Gain Range 0.25 to 5

06 TRANSIENT Filter Magic E GainSweep

It’s a complex response

© 2023 Gain of about 2 “feels” right 41

 Adaptation to Input Filter Inductor

Lfilter = 200 uH

Lfilter = 20 uH

07 TRANSIENT Filter Magic E LfiltSweep

© 2023 42
 Filter Magic Loop Gain

08 AVERAGE SMALL SIGNAL magic loop

This is a strange characteristic.

The dip should be beyond the loop crossover.

Much work to be done – what are the other transfer functions like?

© 2023 43
 Adding Another Power Supply to the System

09 TRANSIENT Filter Magic Two Supplies

11th-order system – easy analysis!
© 2023 44
 Adding Another Power Supply to the System Works Perfectly!

Power supply controlling filter

Second power supply

09 TRANSIENT Filter Magic Two Supplies

Perfect example of Middlebrook’s work in action

© 2023 Look at the impedances to know if this is a rugged system – don’t reanalyze! 45
 Filter Impedance After Control

10 AVERAGE SMALL SIGNAL FilterImpedance with Magic

© 2023 46
 Filter Impedance After Control

Open Loop Input Filter

Closed Loop Input Filter

10 AVERAGE SMALL SIGNAL FilterImpedance with Magic

© 2023 47
 Lots of Work Remains

Students or bored engineers – please pick up the work from here. Call us.

There is enough material in this presentation for an MS/PhD dissertation. Just add equations!

Students – we are happy to work with you on this. (And Professors)

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