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G8MNY > TECH 25.03.24 08:50l 249 Lines 12097 Bytes #999 (0) @ WW
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Subj: Switch Mode PS Principles
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Sent: 240325/0747Z @:GB7CIP.#32.GBR.EURO #:4024 [Caterham Surrey GBR] $:4024_GB
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Dec 06)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
There are 2 types, they work slightly differently (when the secondary diode
conducts), but both use high frequency LF into a ferrite cored transformer.
FERRITE TRANSFORMERS
With ferrite the eddy current losses are very small, & the BH curve magnetic
cycle area which determine the power loss per cycle is also much smaller than
for iron. (difficult to draw here).
___
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 much smaller lighter transformers. For most ferrite
cores the power handling is proportional to 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 ÁÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄ
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. Sometimes a Voltage Dependent Resistor is
placed across the mains circuit in an attempt to protect the components from
line voltage surges.
The turn on current surge is limited by a resistor (say 5R 10W or a thermistor
& sometimes better put in the live feed) that feeds the bridge rectifier into
either 2x 200V electrolytic or 1x 400V one.
BRIDGE
SURGE ÚÄ´>ÃÄÄÄÂÄÄÄÄÄÄÄÂÄÄÄÂÄ +HT ÄÂÄ +HT
FUSE MAINS FILTER LIMIT ³ ³ ³ ³200V ³
LÄÄo-oÄÂÄÄÂÄÄÄ¿ ÚÄÄÄÂÄÄÄÂÄÄRÄÄÁÄ´<ÃÄ¿ ³ 100K === OR WITH ³
³ ³ (((() ³ ³ ³ ³ Link ³ ³ NO 110V ³ 300-380V
Anti ³ === ===== === VDR ÚÄÄÄÄÄÄÄ)Ä)ÄÄo oÄÄÅÄÄÄ´ OPTION === MAINS
shock R ³ (((() ³ ³ ³ ³ ³ 110V ³ ³200V WITH A ³ LIVE HT
NÄÄÄÄÄÄÁÄÄÁÄÄÄÙ ÀÄÄÄÁÄÄÄÁÄÄÄÁÄÂÄ´>ÃÄ)ÄÙ 100K === 400V CAP ³
BALANCED ³ ³ ³ ³ ³
CHOKE ÀÄ´<ÃÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÁÄ -HT ÄÁÄ -HT
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). These high voltage Caps now only have to store the charge
between mains peaks, but also handle the HF ripple load currents from the
following class C switching stage. So the smoothing caps must be low Z at the
pulse frequencies, so low equivalent series resistance ESR caps must be used,
if the mains QRM is to be kept low.
SWITCHING TRANSISTOR
These are normally high voltage NPN or high voltage VMOS FETs nowadays. In
theory there are only ever off or on, so there is almost no power to be
dissipated. But the faster you operate them, to reduce the transformer losses
the slower the edges are, when current at a voltage appears across them
generating heat.
With NPNs a swinging low choke load that presents -ve voltage can turn the NPN
on more than desired (slow to turn off then) so an internal diode is included
in some NPN transistors & can confuse DC testing.
³_ G ÃÄÙD
³/_³_ VMOS ÄÄÄÄÄÄÄÄÄÄÄÂÄ´
NPN Ä´ /_\ ÚÄ´<ÃÄ´>ÃÄÙ Ã>¿S
³\_³ ³ À Ù ³
³e ÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄ´
With VMOS over voltage on the gate drive will destroy the device, external
zeners (15V) are often used "back to back" to clamp the drive, but some devices
may have them internally.
These devices can be damaged by..
1/ too much current, e.g. overloads, sometimes damage is additive over time!
2/ too much voltage, e.g. 500V mains spike, burnt out snubber components.
3/ too hot (too high dissapation/lack of cooling)
4/ under drive causing 3/
5/ overdrive damaging base/gate.
6/ repetative heating (lifetime to 80øC may be 10,000x less than to 50øC)
EMP damage to other components is normal in SMPSUs when they fail & blow fuses!
SINGLE ENDED
These use the back emf or "ringing up" of the half cycle stored energy in the
ferrite cored transformer. The transformer then does 3 jobs, provides
isolation, provides step down ratio, & provide choke input filter. As only DC
pulses are fed into the transformer only half the core BH curve energy can be
utilised.
FAST Pi Output Filter
+HT ÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄ¿ RECTIFIER ====
FERRITE )| ÚÄÄÄÄÄÄÄÄ)ÄÄÄ´>ÃÄÄÄÄÄÄÂÄ(((()ÄÂÄÄÄÂÄÄÄÄÄÄ +
TRANSFORMER )|( high R +³ +³ ³ LOAD
)|( current ³ C1=== C2=== ³
high )| ÀÄÄÄÄÄÄÄÄ)ÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄ)ÄÄÄÄÂÄ -
voltage ___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´I ³ ³ ________ set ³
=== ³ CÄ´ VÃÄÄ)ÄÄÄÄ´ opto ÃÄÄÄ>volts ³
³ R R ÀÄÄÄÂÄÄÄÙ ³ ³isolator³ pot ³
-HT ÄÄÄÄÁÄÄÄÁÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄ´________ÃÄÄÄÄÄÁÄÄÄÄÙ
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 can be
powered from a HT dropper R, or added to by a secondary PS that takes over from
the start up resistor to give enough drive current, or even a separate mains
transformer.
The OSC CR sets the switching frequency, the IC also monitors the transistor
current with a small value emitter R. (If this blows the IC will be distroyed.)
The 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. Sometimes other windings can be used but the regulation may be less
accurate.
To stop the ringing up back EMF from damaging the transistor a snubber CR is
used to terminate the +ve going HF high voltage pulse. Often a minimum load is
needed to keep the size of back emf within safe limits.
Due to the pulse current in the output is it usual to have a pi filter for the
DC output, as a capacitor alone is not too effective.
Rectifier
Secondary³ ringing³__ on ³
voltage ³ up ³ ³ load off ³
³ ³ ³º ringing ³
0v ´ÄÄ¿ pulse³ ³³ÀÄÄÄ¿ ³  Height due
³ ³ width³ º| ³ ³ ³ to mains V &
³ ³______³ ³______³ _³_turns ratio
³TRANSISTOR
³ ON OFF ON
In the single ended SMPSU the diode rectifier only conducts when the transistor
is turned off.
Output filter Caps C1 & C2 must have low ESR (Equivalent Series Resistance)
otherwise they will not only do little filtering but heat up & dry out reducing
their capacitance. This is a common fault with SMPSUs, & on complex equipment
high levels of HF AC ripple on the output DC can cause all sorts of problems.
(Scope the ripple on C1, if higher than 5% of the DC change C1).
PUSH PULL 1
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 rectified & an additional HF pulse choke (about the same size of the
transformer) is now needed to convert the fixed height variable width HF
voltage pulse to the mean output load voltage.
+HT ÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ PULSE optional
ÚÄÄÄ)ÄÄÄÄ¿ diodes R CHOKE pi filter
³ ³ )|| ÚÄ´>ÃÄ¿ ³ ====== ======
³ ³_____)||( ÃÄÄ)ÄÄÄ((((()ÄÂÄ((((()ÄÂÄÄÄÂÄÄÄÄÄÄ +
³ )|| ÃÄÄÄÄÄ)ÄÄ)ÄÄ¿ ³ ³ ³ DC
³ )||( ³ ³ ³ C1=== C2=== ³ OUTPUT
³ ³ ÀÄ´>ÃÄÙ ³ ÀÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄ)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 ÃÄÄÄÄÄÄÄ´I ³ ³ ³ ³
³ ÀÄÄÄÄ)ÄÄÄ´ ³ ³ ³ ________ set ³
=== === ³ CÄ´ VÃÄÄ)ÄÄÄ´ opto ÃÄÄÄ>volts ³
³ ³ R R ÀÄÄÄÂÄÄÄÙ ³ ³isolator³ pot ³
-HT ÄÁÄÄÄÄÄÄÄÁÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄ´________ÃÄÄÄÄÄÁÄÄÄÄÙ
OSC
The control IC now has 2 drive outputs with not more than 45% on time, as each
power switching transistor has to have some time to turn off before the other
antiphase one is turned on.
PUSH PULL 2
There is also a lower voltage design where the half rail voltage (2 Cs) is used
to connect to the output transformer. This means the +ve rail transistor drive
is hot so isolated driver pulse transformers are needed, normally used on both
drives for symmetry.
+HTÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
³ sec \³ In this case current monitoring
=== ((() ÃÄÄ¿ _______ÚÄÄÄÄÁÄÄ¿ may be done on the secondary
³ ===== e/³ )|( ³control³ circuit. Also the need for
³____(((()____³_____)|(__ ³ IC ³ snubbers is less important as
³ FERRITE ³ ³ ³ ³ the output IN THEORY will not
³ TRANSFORMER \³ ³ ³ ³ exceed the power rails.
=== ÃÄÄ¿ ÚÄ)ÄÄÄÄ´ ³
³ e/³ )|( ³ C Ä´ Ã
³ ³ )|( ³ R ÀÄÄÄÂÄÄÄÙ
-HTÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÁÄÄÄÁÄÁÄÁÄÄÄÄÄÄÁÄÄÄÄÄ
PUSH PULL WAVE FORMS 1/2
_____
³ ³ ³
Secondary ³ ³ ringing³T2 on³
voltage ³ º³ ³ ³³
0v ´¿pulseººÀÄÄÄÄÄÄÙ ººÚÄ
³³width³³ º³ringing
³³ ³both off ³
³³_____³
³ T1 on
Voltage ³ _____ _____ _
from ³³ ³ ³ ³ ³ height due
fast ³³_ _ _³ _ _ _ _³_ _ _³ mean ³ to mains V &
diodes ³³ ³ ³ ³ DC ³ turns ratio
ÁÁÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÁÄÄ Á
on to off ratio depends on
load current & supply voltage
The output diodes conduct when the transistors are on, pumping energy into the
pulse choke. As the pulse choke can be made big enough to still give current
between primary pulses (when there is no incoming source voltage so both diodes
may conduct) sometimes a pi DC output 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!
3/ If in doubt if the HT is discharged when off, use a discharge R e.g. 10k 5W.
4/ Use the "one hand behind the back" safety precautions on HT circuits!
FURTHER READING
See my buls on "Stopping HF Tx/Rx SMPS QRM", "Reducing Electronic RF QRM" &
the crowbar part of "High AMP crowbar protected PSU".
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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