Download One valuable thing to understand about the buck circuit is the input

SIMULATIONS WITH THE BUCK TOPOLOGY
EE562 POWER ELECTRONICS I
COLORADO STATE UNIVERSITY
Modified by Colorado State University student
Minh Anh Thi Nguyen
EXTRA CREDIT QUESTIONS
One valuable thing to understand about the buck circuit is the input current from the
source and how we can make it better.
Simulate the following circuit to steady state:
Page 1 of 7
Look at the current from the source V1:
6.0A
4.0A
2.0A
0A
-2.0A
0s
0.1ms
0.2ms
0.3ms
0.4ms
0.5ms
0.6ms
- I(V1)
Time
Page 2 of 7
0.7ms
0.8ms
0.9ms
1.0ms
Look a little closer:
4.0A
3.0A
2.0A
1.0A
0A
400us
- I(V1)
420us
440us
460us
480us
500us
520us
540us
560us
580us
600us
Time
Note: this is only ½ the load current (obviously because the current only flows when the
switch is on…. And this case is a 50% duty cycle).
Just to verify, let’s look at the diode current:
6.0A
4.0A
2.0A
0A
-2.0A
0s
0.1ms
0.2ms
0.3ms
0.4ms
0.5ms
0.6ms
0.7ms
0.8ms
0.9ms
1.0ms
I(D1)
Time
You can see it supplies the rest of the inductor load current.
Now – returning to the current through V1. This choppy current will likely upset other devices
if they are also powered from V1. The current pulses are also a prime source of EMI since the
current can be large and the edge transitions are fast. For both of these reasons, we would add
a filter.
Page 3 of 7
Let’s add an LC filter just to see what it does.
Now looking at the input current from V1 and we see:
6.0A
4.0A
2.0A
0A
-2.0A
0s
0.5ms
1.0ms
1.5ms
2.0ms
2.5ms
3.0ms
3.5ms
4.0ms
- I(V1)
Time
Why is this ringing like this? What is difference between the input current before
and after the filter? Can we change the frequency of this ringing? How?
Page 4 of 7
Now let’s add a typical ESR to the input capacitor and see what happens:
4.0A
2.0A
0A
-2.0A
0s
0.5ms
1.0ms
1.5ms
2.0ms
2.5ms
3.0ms
3.5ms
4.0ms
- I(V1)
Time
The ringing is damped this time. What are other ways the ringing could be damped? (The
inductor likely will have some resistance in the wires) Why would we WANT the ESR for
this cap? (Instead of a ceramic with extremely low ESR)
Page 5 of 7
Now let’s look at the current in the inductor and the current in the capacitor:
4.0A
0A
SEL>>
-4.0A
I(L7)
4.0A
0A
-4.0A
0s
0.5ms
1.0ms
1.5ms
2.0ms
2.5ms
3.0ms
3.5ms
4.0ms
I(C6)
Time
A little closer:
1.50A
1.25A
1.00A
I(L7)
4.0A
0A
SEL>>
-4.0A
2.00ms
I(C6)
2.05ms
2.10ms
2.15ms
2.20ms
2.25ms
2.30ms
2.35ms
2.40ms
2.45ms
2.50ms
Time
Note: The inductor current is the same as the current from V1. We can say that the inductor
supplies the DC current and the capacitor is delivering the AC current. So – how would we
select the component ratings for this filter? Hint: Let’s change the traces to look at the
Irms of the capacitor and the Iavg (average current) of the inductor:
2.0A
1.0A
0A
AVG(I(L7))
4.0A
2.0A
SEL>>
0A
0s
0.5ms
1.0ms
1.5ms
2.0ms
2.5ms
RMS(I(C6))
Time
Page 6 of 7
3.0ms
3.5ms
4.0ms
The steady state average current through the inductor is 1.17A
The steady state rms current through the capacitor is 1.4Arms.
Inductors are sized based on the max DC current. We would probably add some margin and
pick an inductor that was at least capable of 2A, maybe more depending how conservative we
want to be.
Capacitors (aluminum electrolytics) data sheets specify the maximum ripple current rating
(Irms). This can also be called the self heating loss. The lifetime of the part will depend on the
Irms, the ESR, the package size, and the temperature.
For this capacitor, we would want to choose the Irms rating at least at 2A. We could use two
capacitors with each having an RMS rating of 1A. This would depend on how much the
capacitors cost and what is available.
Find a capacitor that would work in our input filter: (Hint, Rubycon ZL series has some. You
can find the specification sheet at: www.rubycon.com). Make sure that the rated voltage of the
part is at least 1.5 times the input voltage (or at least 36V). Some people use 2x.
I found a Rubycon ZL 470 cap:
V=50V
C=470uF
Irms=2050mA
ESR=27mohms
What changes when we put this cap in vs. the 100uF cap we initially started with?
(Hint: resonant frequency, Irms changes a bit due to lower ESR)
What is the input voltage at the top of the switch? (It isn’t perfect DC anymore --- which
in real life it never is). How will that affect our converter and the output? Look at the
output voltage ripple at steady state and compare to the peak to peak ripple you measured on
page 6 (question 3). Is there any difference?
12.69V
12.00V
11.00V
10.01V
3.60ms
3.62ms
V(C1:1)
3.64ms
3.66ms
3.68ms
3.70ms
Time
Page 7 of 7
3.72ms
3.74ms
3.76ms
3.78ms