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G8MNY > TECH 19.12.05 19:43l 162 Lines 7774 Bytes #999 (0) @ WW
BID : 9302_GB7CIP
Read: GUEST DL1LCA OE7FMI
Subj: Switch Mode PS Principles
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Sent: 051219/1027Z @:GB7CIP.#32.GBR.EU #:9302 [Caterham] $:9302_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (New Dec 05)
There are 2 types, they work slightly differently but both use high frequency
LF into a ferrite cored transformer. With ferrite the eddy current losses are
very small the BH curve magnetic cycle area loss is also much smaller than for
iron.
__
IRON ³ __.--"/ A MODERN ³ ..-'
LAMINATE ³./' ./' FERRITE ³ //
./³ ./' ³ //
ÄÄÄÄ/ÄÄÅÄÄ/ÄÄÄ +H ÄÄÄÄÄÄ/Å/ÄÄÄÄÄ +H
./' ³/' // ³
./' _./'³ // ³
__.:..-" ³-B .-'' ³-B
The high frequency means smaller lighter transformers. In fact for most
ferrites the power handling is proportional to the frequency. So a core running
at 250KHz can handle say 250W, but at 50KHz only 50W.
Transformer 8 ´ _ . -"""-._ eddy current &
Power 4 ´ _ . - ' `-. BH losses
2 ´ . - ' \ win out
1 ÁÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄ
5 10 20 40 80 160 320 640 KHz
The problems at these higher frequencies are the other circuit losses, not just
the increased eddy current & BH losses in the ferrite, but inter winding
capacitance, winding skin effect copper losses etc. External losses also
increase, such as transistor on & off times (FETs can be fast if driven well),
& diode rectifier on & off times & of course not to forget the increased
radiation losses from all the wiring as the frequency goes up.
MAINS CIRCUIT
Both types of SMPSU start with a MAINS FILTER (not always fitted despite QRM
laws!) the capacitors must be the X rated type for mains work if earthed there
will also be 4n7 caps to earth. The surge limiting resistor (say 5R 10W or a
thermistor) feeds the bridge rectifier into either 2x 200V electrolytics or 1x
400V one. With the 2 caps a 110v mains option can be provided with just a wire
link, that uses only half the bridge as a voltage doubler. (OK if mains 60Hz &
the 2 caps are large value).
BRIDGE
SURGE ÚÄ´>ÃÄÄÄÂÄÄÄÄÄÄÄÂÄÄÄÂÄ +HT ÄÂÄ +HT
FUSE MAINS FILTER LIMIT ³ ³ ³ ³200v ³
LÄÄo-oÄÂÄÄÂÄÄ¿ ÚÄÄÄÂÄÄÄRÄÄÄÄÁÄ´<ÃÄ¿ ³ 100K === OR WITH ³
³ ³ (((() ³ ³ ³ Link ³ ³ NO 110V ³ 300-380V
Anti ³ === ===== === ÚÄÄÄÄÄÄÄÄ)Ä)ÄÄo oÄÄÅÄÄÄ´ OPTION === MAINS
Shock R ³ (((() ³ ³ ³ ³ 110V ³ ³200v WITH A ³ LIVE HT
NÄÄÄÄÄÄÁÄÄÁÄÄÙ ÀÄÄÄÁÄÄÄÄÄÁÄÄÂÄ´>ÃÄ)ÄÙ 100K === 400V CAP ³
BALANCED ³ ³ ³ ³ ³
CHOKE ÀÄ´<ÃÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÁÄ -HT ÄÁÄ -HT
SINGLE ENDED
These use the back emf or "ringing up" of the stored energy in the ferrite
cored transformer. The transformer then does 3 jobs, provides isolation,
provides step down ratio, & provide choke input filter.
FAST Pi O/P Filter
+HT ÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄ¿ RECTIFIER ====
FERRITE )|ÚÄÄÄÄÄÄÄÄÄÄ)ÄÄ´>ÃÄÄÄÄÄÄÄÂÄ(((()ÄÂÄÄÄÂÄÄÄÄÄÄ +
TRANSFORMER )|( R === === ³ LOAD
³ ÀÄÄÄÄÄÄÄÄÄÄ)ÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄ)ÄÄÄÄÂÄ -
___³ ÃÄÄÄÄ¿ ³ ³
³ \³ ÚÄÄÄÄÄÁÄ¿ ³+ ³ ³
R ÃÄÄÄÄ´Control³ === ³ ³
SNUBBER ³ e/³ ³ IC ³ ³ ³ ³
³ ÃÄÄÄÄÄÄ´ ³ ³ ________ set ³
=== ³ C ´ ÃÄÄ)ÄÄ´ opto ÃÄÄÄÄÄ>volts ³
³ R R ÀÄÄÄÂÄÄÄÙ ³ ³isolator³ pot ³
-HT ÄÄÄÄÁÄÄÄÁÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄ´________ÃÄÄÄÄÄÄÄÁÄÄÄÄÙ
OSC
The phase of the transformer is arranged so that when the transistor is turned
off the rectifier on the secondary conducts. The pulse width control IC is
powered from a HT dropper R, sometimes there is a separate PSU or secondary
that takes over for this.
The OSC CR sets the switching frequency, the IC also monitors the transistor
current with a small value emitter R. Voltage feedback from the output drives
an opto isolator to control the pulse width of the IC to alter the energy
stored in the transformer for the next pulse. As only DC pulses are fed into
the transformer only half the core BH curve energy can be.
Secondary³ Rect__on
voltage ³ ³ ³ ringing
³ ³ ³º
0v ´Ä¿pulse³ ³³ÀÄ¿  Height due
³ ³width³ º³ ³ ³ ³ to mains V &
³ ³_____³ º ³_______³ _³_turns ratio
³TRANSISTOR pulse width
³ ON
Due to the pulse current in the output is it usual to have a pi filter for the
DC output.
PUSH PULL
These are more complex & more powerful. The double action utilises the full
power handling of the transformer core, but that means the secondary must be
full wave & an additional HF choke (about the same size of the transformer) is
now needed is needed to convert the HT fixed height variable width voltage
pulse to a variable current feed to the secondary load.
The control IC now has 2 drive outputs with not more than 45% on, as each power
switching transistor has to have some to turn off before the other antiphase
one is turned on.
+HT ÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ PULSE optional
ÚÄÄÄ)ÄÄÄÄ¿ diodes ³ CHOKE pi filter
³ ³ )|| ÚÄ´>ÃÄÂÄÄ)ÄÄÄ¿ ===== ====
³ ³_____)||( ³ R ((((()ÄÄÂÄ((((()ÂÄÄÄÂÄÄÄÄÄÄ +
³ )|| ÃÄÄÄÄÄ)ÄÄ)ÄÄ¿ ³ ³ ³ DC
³ )||( ³ ³ ³ === === ³ OUTPUT
³ ³ ÀÄ´>ÃÄÙ ³ ÀÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄ)ÄÄÄÄÂÄ -
³ ___³ ÃÄÄÄÄ¿ ³ ³
³ ³ \³ ÚÄÄÄÄÄÁÄ¿ ³+ ³ ³
__³ R T2ÃÄÄÄÄÄ´Control³ === ³ ³
³ \³ ³ e/³ ³ IC ³ ³ ³ ³
R T1ÃÄÄ)ÄÄÄ)ÄÄÄÄÄÄÄ´ ³ ³ ³ ³
³ e/³ ³ ³ ³ ³ ³ ³ ³
³ ÀÄÄÄÄ)ÄÄÄÅÄÄÄÄÄÄÄ´ ³ ³ ________ set ³
=== === ³ C ´ ÃÄÄ)ÄÄ´ opto ÃÄÄÄÄÄ>volts ³
³ ³ R R ÀÄÄÄÂÄÄÄÙ ³ ³isolator³ pot ³
-HT ÄÁÄÄÄÄÄÄÄÁÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄ´________ÃÄÄÄÄÄÄÄÁÄÄÄÄÙ
OSC
______
³ ³ ³
Secondary ³ ringing ³T2 on ³
voltage ³ º³ ³ ³
0v ´¿pulseººÀÄÄÄÄÄÄÙ ººÚÄ
³³width³ º³
³³ ³ ringing
³³_____³
³ T1 on
Voltage ³ ______ ______ _
from ³³ ³ ³ ³ ³ height due
³³ _ _ _³ _ _ _ _³ _ _ _³ mean ³ to mains V &
fast ³³ ³ ³ ³ DC ³ turns ratio
diodes ÁÁÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄ Á
on to off ratio depends
on load & supply voltage
As the pulse choke can be made big enough to still give current between primary
pulses sometimes a pi DC filter is not used.
SAFETY
1/ Working on them it is recommended you use a mains isolation transformer.
2/ Use a high voltage 1/10 or 1/100 scope probe, rated at 600V @ 100KHz not one
rated at 400v @ 100Hz MAX!
3/ If in doubt if HT is discharged when off use a discharge R 10K 5W.
4/ Use the 1 hand behind the back safety precautions on HT circuits!
FUTHER READING
See buls on "Stopping HF Tx/Rx SMPS QRM", "Reducing Electronic RF QRM" & the
crowdbar part of "High AMP crowbar protected PSU".
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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