SWITCH-MODE POWER SUPPLIES AND SYSTEMS

1
SWITCH-MODE POWER SUPPLIES AND SYSTEMS
Lecture No 10 Switching transformer design
rules. Power losses analysis in switching
regulators

• Silesian University of Technology
• Faculty of Automatic Control, Electronics
• and Computer Sciences
• Ryszard Siurek Ph.D., El. Eng.

2
Flyback converter transformer
IT
I0
D1
Ipmax
IC
R0
UIN
C
Zp
ZS
U0
B
t
IT
BS
T
energy storing during cycle I
DB
H
Minimum number of zP turns assuming t tmax, DB
Bs, UIN UINmax
Assuming required output power equal to P0
zpmin is set for the chosen core
Certain air-gap is necessary to achieve required
output power
3
Cycle II - transistor T is off
ID
D1
ID
I0
IDmax
IC
T
R0
C
ZS
t
B
t
U0
BS
Energy stored in the core is trans-fered to the
output during cycle II
H
Selecting t lt T-t for the maximum output
power Po one decides to work with the
discontinuous magnetic flux flow in the
whole range of load changes. To increase t one
must also increase LS, and it is related to
higher number of turns of the secondary winding
zS. When t T-t transformer starts to
operate with continuous magnetic flux flow.
For discontinuous flux flow
For continuous flux flow
4
Flyback transformer design simplified procedure

1. Select maximum (nominal) output power Po
2. Select switching frequency basing on
   specifications of available magnetic material,
   semiconductors etc.
3. Calculate tmax, current Imax and required value
   of Lp
4. Select core dimensions accordind to Hahn
   diagrams or using AP method (same as in
   inductor design procedure)
5. Calculate (find from diagrams) the air-gap
6. Calculate minimum number of primary turns,
   calculate required number of turns zP
7. Select operating pronciple (continuous or
   discontinuous flux flow)
8. Calculate secondary number of turns zS

• Usually discontinuous flux flow is observed in
  flyback converters due to the following reasons
• lower number of winding turns (lower copper
  power losses)
• lower level of EMC disturbances (transistor
  switches on with current equal to 0)
• self-oscillating converter is very easy to
  design (low-cost solution)

5
Forward converter transformer
Ip
Ip
IS
IM
UIN
Zp
ZS
US
Lp
transformer
equivalent circuit

1. Selection of the core - basing on diagrams
   (nomograms etc.) relating core dimensions to
   total power for certain converter topology
2. Calculation of minimum number of turns for the
   primary winding to avoid saturation in most
   unfavourable operating conditions

Equation identical for any converter
topology

1. Selection of wire cross section (diameter) taking
   into account primary current RMS value and
   calculation of number of turns for required
   winding inductance Lp (using Al constant for
   selected core) the following condition must be
   performed zp gt zpmin

6

1. Calculation of secondary winding (windings)
   number of turns

1. Calculation of wire cross section area (copper
   strip, litz wire) for secondary winding resulting
   from secondary current RMS value ISrmsnIprms
2. Checking if it is enough space to place windings
   in the core (bobbin) window area required
   isolation and winding arrangement according to
   safety standards must be considered

bobbin
secondary windind
magnetic core
leakage distance (6 mm)
Safety insulation (3 layers)
primary winding
functional insulation (between winding layers)
3 mm
7
General notes

1. Core power loses are higher when frequency and
   flux density amplitude increase - that is why
   the high value of primary inductance Lp is
   desirable
2. High Lp value is related to more primary turns
   more trouble with placing the winding in the
   bobbin and higher copper losses - look
   for optimum settlemet!
3. Chose the magnetic core with best available
   performance high saturation flux density Bs,
   lowest power losses, smallest dimensions
4. Small air-gap in transformer core may be
   considered (forward converter) better
   utilisation of the core may be achieved by
   lowering magnetic remanence

B
DB - without air-gap
DB - with small air -gap
H

1. Remember that Bs value decreases with temperature
   at 100oC it is lower by 20 25 in
   comparison to the value specified at 25oC

8
Switching regulator power losses analysis
1. Switching power losses (dynamic)
L
LL
UT
IL
ILmax
I0
IL
I0
IT
ID
T
UIN
D
Ro
C
t
T
Ucontr
t
0
ILmax
IT
ILmin
ts
QR - diode reverse charge mC
ILmax
td
tf
,
ID
t1
t1
ILmin
overvoltage due to leakage inductance
UT
-IRmax
UIN
ITrmsrds
Discharging of transistor capacitances CBCand CBE
(bipolar transistors)
Eloss
9
Swith-mode power supply power losses - review

• Power losses in passive components
• - winding resitances (skin and proximity
  effects)
• - capacitor series resitance (ESR) output
  filer electrolytic capacitors
• - magnetic core losses (hysteresis and eddy
  currents)
• - power losses in snubbar circuits
• Static power losses in semiconductors
• - related to ON- resistance of MOSFET
  transistor or saturation voltage drop
• across bipolar transistor
• - related to voltage drop across rectifier
  diodes (mains input rectifier)
• and fast swiching output diodes
• IMPORTANT!
• for bipolar transistors and diodes
  for MOSFET trnasistors
• Switching (dynamic) power losses
• - related to semiconductor switching times,
  reverse charges
• - depandant on base (gate) drive circuits

10
How to minimize power losses general rules

• Power losses in passive components
• - select proper wire diameter, use copper
  stripes or litz wire
• - select low ESR capacitors (for switching
  applications), as big (dimensions)
• as possible, connected in parallel,
• - make wide and thick copper paths on the PCB
• - select modern ferrite cores with best
  performance at specified operating
• frequency and smallest dimensions
• - avoid high amplitude of flux density changes
• - recover the magnetising energy do not
  dissipate it
• - use converter topologies with low
  overvoltages decrease the influence of
• leakagae inductances (eg. two-transistor
  forward topology)
• Static power losses in semiconductors
• - select MOSFET trnasistors with low
  on-resistance
• - in high power and high voltage applications
  use IGBT modules (simple
• MOSFET drive circuits, low saturation
  voltage drop as for bipolar transistors)
• - use Shottky diodes if possible (voltage drop
  below 0,5V)
• - use synchronous rectification technique

After switching on the internal body diode the
transistor with very low on-resistisance switches
on the voltage drop across the conducting
transistor is much lower than across the diode
11

• Transistor dynamic power losses
• - select fast transistors (low tr anf tf times)
• - use special converter topologies with
  zero-current or zero-voltage switching
• (eg. resonant topologies)
• - design carefully snubbar circuits

Voltage UT rise without snubbar circuit
IT
Zp
UT
CIN
Charging of the capacitor delays voltage rise
across the switching transistor decreasing
significantly transistor power losses
UIN
Cs
IT
UT
T
t
Ds
It is possible to select such value of the
capacitor Cs, that the overall power loss in the
transistor and snubbar circuit reaches minimum.