热搜关键词: Protel 99SE欧姆龙PLCGD32变压器

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PI设计指导

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标签: 电子电路

电子电路

PI设计指导PI设计指导PI设计指导

Power Integrations - Power Puzzler 1
POWER
puzzler
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Understanding the Effects of Diodes
by Peter Vaughan
Manager of Product Applications
Power Integrations
The schematic below shows a Flyback power supply built with a
TOPSwitch
®
-GX
power conversion IC. The following questions concern the selection of the
output diode (D3) and the clamp diode (D1).
Question 1: beginner
Which diode or diodes from the following list are suitable for use as D3 and why?
a.
b.
c.
d.
e.
f.
80 V, 5 A, Schottky (e.g. SB580)
600 V, 3 A, Ultrafast (e.g. UF5406)
50 V, 3 A, Rectifier (e.g. 1N5400)
100 V, 8 A, Ultrafast (e.g. BYV29-100)
45 V, 7.5 A, Schottky (e.g. MBR745)
1000 V, 2.5 A, Fast (e.g. FR257)
Click to see the Answer
Input: 90-375 VDC
Output: 12 V, 2.5 A
http://www.powerint.com/powerpuzzler050308/(第 1/8 页)2007-8-9 2:08:09
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Power Integrations - Power Puzzler 1
Answer:
a and d
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Suitability of Diode Parameters
Diode
(a) SB580
(b) UF5406
(c) 1N5400
Diode Parameters
80 V, 5 A
600 V, 3 A, 75 ns
50 V, 3 A, 2000 ns
Peak Reverse
Voltage (V
RRM
)
X
X
-
Average Forward
Current (I
AVE
)
X*
-
-
Reverse Recovery
Time (t
rr
)
X
-
-
(d) BYV29-100
(e) MBR745
(f) FR257
100 V, 8 A, 25 ns
45 V, 7.5 A
1000 V, 2.5 A, 500 ns
X
-
X
X
X
-
X
X
-
* With bench verification of temperature rise
Why?:
In selecting the output rectifier three key parameters should be considered; voltage rating, current rating and reverse recovery time.
Considering each of these in turn we can see which of the diodes is suitable.
Maximum repetitive reverse voltage (V
RRM
)
Ideally the diode selected should have a V
RRM
rating 1.25·V
R
, where V
R
is the reverse voltage seen by the diode. The 1.25 de-rating factor provides margin to take
account of leakage inductance generated voltage spikes, and AC line transients that increase the DC bulk voltage temporarily plus reducing the device stress improves
reliability.
Normally where people get into trouble is determining the value of V
R
. You’d be forgiven for thinking that the reverse voltage seen by the diode is just the output
voltage, here 12 V so anything above a 15 V diode would be fine. However the anode of D3 is connected to a Flyback transformer. This means that when the MOSFET
inside U1 turns on, under maximum input line of 375 VDC (265·VAC x √2) is seen across the primary. This voltage is transformed by the primary to secondary
turns ratio and due to the phase of the windings drives the anode of the output diode negative. In this design the value of V
R
is given by:
V
R
= V
O
+ (V
MAX
)·(N
S
/N
P
)
= 12 + 375·6/58
= 50.7 V
http://www.powerint.com/powerpuzzler050308/(第 2/8 页)2007-8-9 2:08:09
这个公式是很重要的,在MOSFET导通的瞬间,
初级有一个电势会耦合到次级, 和输出滤波电容器两端
电压成串联叠加 D3
两端。
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Power Integrations - Power Puzzler 1
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Applying the 1.25 V de-rating this gives a minimum diode rating of 63 V. The closest standard diode rating would be an 80 V Schottky or a 100 V PN diode.
Therefore all the diodes have an acceptable V RRM rating except (c) and (e).
Forward Current Rating (I
AVE
)
Diodes manufacturers generally specify the current rating as the average current through the diode or for a Flyback power supply the load current. In this design the
specified output current is 2.5 A so a diode with a current rating above 2.5 A is acceptable right? Yes and no.
这句话的意思是1N54系列:
从厂商提供的降额曲线来看3A
的管子在引线周围温度为100摄氏度时实际上最多只½通过1.5A
的平均电流。
The diode manufacturers provide de-rating curves so for example at a lead temperature of 100 °C a 3 A PN diode is actually a 1.5 A diode!
Diode type. The choice between PN and Schottky and the actual voltage rating changes a diode’s forward voltage. The lower the forward voltage drop, the
lower the dissipation is for a given forward current.
Overload fault conditions. All power supplies have some degree of overload capability. Due to tolerances a typical supply will be able to deliver significantly
more than the specified output power, especially at high line. Therefore if an overload condition can exist then the diode should be sized to operate without
overheating and failing. Products from Power Integrations significantly help here as the key parameters associated with power delivery (frequency and current
limit) are very tightly specified, reducing the overload power. In addition line voltage power limiting is easy to add to reduce overload power. In this example the
addition of R2 reduced the primary current limit as the line voltage increases and provides a very flat overload power characteristic with line voltage as shown
below.
As a general rule of thumb select the current rating such that I
AVE
= 3 x I
O
irrespective if the diode is a PN or a Schottky type (although a Schottky diode will have lower
dissipation check the maximum operating temperature – some are 25-50 ° C lower than a PN diode). Finally measure the diode temperature under maximum
overload power, highest ambient to check it is within either internal or manufacturers design limits.
Looking at the diodes in the list to determine if they are acceptable or not:
Rated
I
AVE
5A
Diode
(a) SB580
Acceptable?
Notes
Yes - based on bench I
AVE
is only 2 x I
O
it’s a Schottky diode has a
verification
maximum temperature specification of 150 C
and used in a supply with power limiting (R2
and R4 in schematic) so it’s likely this diode
would be acceptable.
No
No
No
I
AVE
is only 1 or 1.2 x I
O
- diodes would
overheat
(b) UF5406
(c) 1N5400
(f) FR257
3A
3A
2.5 A
http://www.powerint.com/powerpuzzler050308/(第 3/8 页)2007-8-9 2:08:09
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Power Integrations - Power Puzzler 1
(d) BYV29-100
8A
Yes
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I
AVE
is ≥3 x I
O
and in a TO220 package. When
attached to a suitable heatsink this diode is a
good choice.
So diodes (a), (d) and (e) have an acceptable current rating but if you ruled out (a) that would also be ok.
Reverse recovery time (t
rr
)
An ideal diode would instantly block reverse current flow when a reverse bias is applied. In practice a Schottky approaches zero recovery time however for a PN diode
it takes a finite time for charge stored in the diode to be swept away before it can block. The amount of charge is proportional to the current flowing through the diode.
This is significant as even a Flyback converter operating in discontinuous conduction mode will actually operate in continuous conduction mode at start-up meaning
the output diode has to reverse recover with a significant forward current flowing. In this design the converter operates in continuous conduction mode some the diode
has to recover with forward current on every switching cycle.
To prevent large reverse currents from flowing through the diode when the MOSFET in U1 turns on, applying a reverse voltage, the diode should have a reverse
recovery time less than switching time. This means a recovery time of 50 ns or better.
Longer duration than this causes large primary side current spikes at the turn on event and high diode dissipation – for example the 1N5400 became hot enough to
melt the solder holding into the PCB while taking the measurements for question 2.
Diode
(a) SB580
(b) UF5406
(c) 1N5400
(f) FR257
(d) BYV29-100
(e) MBR745
t
rr
0
75 ns
-
500 ns
25 ns
0
Acceptable?
Yes
No
No
No
Yes
Yes
Not specified but >>2000 ns
Notes
So diodes (a), (d) and (e) have an acceptable reverse recovery time.
http://www.powerint.com/powerpuzzler050308/(第 4/8 页)2007-8-9 2:08:09
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Power Integrations - Power Puzzler 1
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Question 2: advanced
Below is a series of oscillograms showing the drain voltage (V
DS
) and current (I
D
) of U1, measured with the different diodes listed in question 1*. Can you match
the resultant waveforms to the diode(s) that generated them?
Waveform 1: ________
Waveform 2: ________
Waveform 3: ________
Waveform 4: ________
*Test Conditions: 1.5 A load @ 115 V AC; Upper Trace V
DS
=100 V/div; Lower Trace I
D
=0.4 A/div, 2
µs/div
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