BM2P0XX series PWM Flyback converter Technical Design

PWM type AC/DC converter IC with Built-in 650V MOSFET
BM2P0XX series PWM Flyback converter Technical Design
This application note describes the design of the PWM flyback converters using ROHM’s AC/DC converter IC BM2Pxxx series
devices. It explains the selection of external components and provides PCB layout guidelines. Please note that all performance
characteristics have to be verified. They are not guaranteed by the PCB layout shown here.
● Description
The BM2Pxxx series of ICs are AC/DC converters for PWM switching, incorporating a built-in starter circuit having withstanding
voltage of 650V and a switching MOSFET having withstanding voltage of 650V. With ROHM’s original high-speed switching
MOSFET built inside, it is possible to increase the peak current, contributes to miniaturization of the magnetic components.
BM2Pxxx supports both isolated and non-isolated circuits, enabling simpler design of various types of low-power converters.
● Key features
- PWM frequency 65kHz (with frequency-hopping function)/ Current mode
- Burst-operation and frequency reduction functions when load is light
- Built-in 650V starter circuit / Built-in 650V switching MOSFET
- VCC pin under-voltage protection/Over-voltage protection
- SOURCE pin Open/ Short protection, Leading-Edge-Blanking function
- Per-cycle over-current limiter function
- Over-current limiter AC correction function
-
Soft-start function
● BM2Pxxx Series line-up
Function
Brownout
VCC OVP
BM2P051F
Latch stop
Yes
BM2P052F
Auto restart
5.5Ω
2.6A
8W
BM2P053F
Latch stop
BM2P054F
Auto restart
SOP8
BM2P091F
Latch stop
Yes
BM2P092F
Auto restart
12Ω
1.3A
5W
BM2P093F
Latch stop
BM2P094F
Auto restart
BM2P011
Latch stop
Yes
BM2P012
Auto restart
2.0Ω
10.4A
20W
BM2P013
Latch stop
BM2P014
Auto restart
Latch stop
BM2P031
Yes
BM2P032
Auto restart
3.6Ω
5.4A
15W
BM2P033
Latch stop
BM2P034
Auto restart
DIP7
BM2P051
Latch stop
Yes
BM2P052
Auto restart
5.5Ω
2.6A
10W
BM2P053
Latch stop
BM2P054
Auto restart
BM2P091
Latch stop
Yes
BM2P092
Auto restart
12Ω
1.3A
7W
BM2P093
Latch stop
BM2P094
Auto restart
*1 These are reference values in case of PWM Flyback converter. It is necessary to limit output power depending on power
Product
Package
MOSFET
RDS(ON) (max)
IDP(max)
Max Output Power *1
85-265Vac
supply specification.
www.rohm.com
© 2013 ROHM Co., Ltd. All rights reserved.
1/8
Oct. 2013 - Rev.A
BM2P0XX series PWM Flyback converter Technical Design
Application Note
1. Design Example of Isolated Type Flyback Converter DCM (Discontinuous Conduction Mode)
Figure 1-1.Isolated Type Flyback Converter Circuit Example
Basic operation of flyback converter
(1) When switching is turned ON
Np
VIN
Ns
(2) When switching is turned OFF
OFF
Lp
Ls
Lp
Np
VIN
Ls
Ip
ON
Ns
ON
Is
OFF
When MOSFET is ON, current Ip flows through the
When MOSFET is OFF, the accumulated energy is
transformer’s primary-side winding Lp, and energy is
output from the secondary-wide winding Ls, current Is
accumulated.
flows via the diode.
At that time, the diode is off.
Ip 
Np
VO
 Ip 
 toff
Ns
Ls
Ns ton
VO 

 VIN
Np toff
VIN
 ton
Lp
www.rohm.com
© 2013 ROHM Co., Ltd. All rights reserved.
Is 
2/8
Oct. 2013 - Rev.A
BM2P0XX series PWM Flyback converter Technical Design
Application Note
1-1. Transformer T1 design
1-1-1.
Determination of flyback voltage VOR
Flyback voltage VOR is determined along with turns-ratio Np:Ns
and duty-ratio.
Np ton

 VIN
Ns toff
Np VOR


Ns
VO
VOR
 Duty 
VIN  VOR
VOR  VO 
VIN→
When VIN = 95V (AC 85V x 1.4 x 0.8), VOR = 65V, Vf = 1V:
VOR
GND→
Np VOR
VOR
65V



5
Ns VO Vout  Vf 12V  1V
VOR
65V
Duty(max)

 0.406
VIN(min) VOR 95V  65V
Figure 1-2. MOSFET Vds
(*) When duty is 0.5 or above, VOR is adjusted to set it below 0.5.
1-1-2. Calculation of secondary-side winding inductance Ls and
secondary-side maximum current Ispk
For better power efficiency, if Iomax = Io x 1.2 = 1.2A:
Ls＜
Vout  Vf   1 - Duty2
2  Iomax  fswmax
12V  1V  1 - 0.4062  27.3uH

2  1.2A  70kHz
Ispk 
2  Iomax
2  1.2A

 4.04A
1 - Duty(max) 1 - 0.406
Figure 1-3. Primary-side and Secondary-side Current Waveforms
1-1-3. Calculation of primary-side winding inductance Lp and primary-side maximum current Ippk
2
 Np 
2
Lp  Ls  
  27.3uH 5  683uH
 Ns 
Ippk  Ispk 
Ns
1
 4.04A   0.81A
Np
5
1-1-4. Determination of transformer size
Based on Po = 12W, the transformer’s core size is EI22.
Table 1-1. Output Voltage and Transformer Core
Core sectional area
Ae (mm2)
~5
EE13
16
~10
EI19/EE19
23
~20
EI22/EE22
37
(*) The above are guideline values. For details, check with the transformer manufacturer, etc.
Output voltage Po (W)
www.rohm.com
© 2013 ROHM Co., Ltd. All rights reserved.
Core size
3/8
Oct. 2013 - Rev.A
BM2P0XX series PWM Flyback converter Technical Design
Application Note
1-1-5. Calculation of primary-side turn count Np
Np 
VIN ton Lp  Ippk

Ae Bsat Ae Bsat
Generally, the maximum magnetic flux density B(T) for an ordinary ferrite core is 0.4T @100°C, so Bsat = 0.3T.
Np 
Lp  Ippk
683uH  0.81A

 49.8 turns  Np is 50 turns or above
Ae  Bsat
37mm 2  0.3T
Since magnetic saturation does not result from this, Np is set based on the AL-value－NI characteristics.
When AL-value = 150 nH/turns2 is set,
Np 
Lp
683uH

 67.5turns　 68 turns
AL - Value
150nH/turns 2
NI  Np  Ippk  68turns 0.81A  55.1A･turns
The AL-value－NI characteristics of EI22 are used to confirm that this is within the tolerance range.
When it is beyond the tolerance range, Np is adjusted.
AL-Value=150nH/turns
2
NI=55.1A・turns
Figure 1-4. EI22 AL-value vs. NI Limit Characteristics (Tomita 2G8-EE22)
1-1-6. Calculation of secondary-side turn count Ns
Np
68
 5  Ns 
 13.6 turns  14 turns
Ns
5
1-1-7. Calculation of VCC turn count Nd
When VCC = 15V, Vf_vcc = 1V,
Nd  Ns 
VCC  Vf_vcc
15V  1V
 1４turns 
 17.2turns　 17 turns
Vout  Vｆ
12V  1V
As a result, the transformer specifications are as follows.
Table 1-2. Transformer Specifications
Core
Tomita 2G8-EI22/EE22 or compatible
Lp
683 uH
Np
68 turns
Ns
14 turns
Nd
17 turns
www.rohm.com
© 2013 ROHM Co., Ltd. All rights reserved.
4/8
Oct. 2013 - Rev.A
BM2P0XX series PWM Flyback converter Technical Design
Application Note
1-2. Selection of main components
1-2-1．IC1
Since Pout = 12V × 1A = 12W, BM2P034 is selected.
1-2-2. Input capacitor: C1
Use Table 1-3 to select the capacitance of the input capacitor.
Since Pout = 12V × 1A = 12W, C1 = 2 × 12 = 24  33μF.
Table 1-3. Input Capacitor Selection Table
Input voltage (Vac)
Cin (μF)
85-264
2 X Pout(W)
180-264
1 x Pout(W)
(*) The above values are guidelines for full-wave rectification. When selecting, also consider other specifications
such as the retention-time.
The withstanding voltage of the capacitor becomes, Vac (max) × 1.41. Say for AC 264V, it is 264V × 1.41 = 372V, so this
should be 400V or more.
1-2-3. Current-sensing resistor: R1
The current-sensing resistor limits the current that flows on the primary side to provide protection against output overload,
and is used for slope compensation of current mode control. Consequently, in some cases limits may be imposed according
to the transformer’s primary-side inductance and input voltage.
In the BM2P0XX Series, an AC voltage correction function is built-in the chip for overload protection. This corrects offsetting
of the overload protection point caused by different input voltages (such as AC 100V and AC 200V).
Vcs_limit Vcs  ton  20mV/us
R1 


Ippk
Ippk
Vcs 
Duty
0.406
 20mV/us
0.4V 
 20mV/us
fsw
65kHz
　 0.64 Ω　　0.56 Ω
Ippk
0.81A
Confirm the overload protection point while the resistor is assembled in the product.
Sensing resistance loss P_R1:
P_R1(peak)  Ippk２  R1  0.812  0.56  0.37W
２
２


Duty 
0.406 
  R1   0.81
  0.56  0.05W
P_R1(rms)  Iprms２  R1   Ippk 


3 
3 


Set to 0.5W or above in consideration of pulse resistance.
With regard to pulse resistance, the structure of the resistance may vary even with the same power rating.
Check with the resistor manufacturers for details.
1-2-4. VCC-diode: D2
A high-speed diode is recommended as the VCC-diode.
Reverse voltage applied to the VCC-diode:
Vdr  VCC(max)＋VINmax
Nd
Np
When VCC (max) = 29 V,
Vdr  29V＋374V 
15
 122.5V　60
With a design-margin taken into account, 122.5V / 0.7 = 175V  200V component is selected.
(Example: ROHM’s RF05VA2S 200V, 0.5A)
www.rohm.com
© 2013 ROHM Co., Ltd. All rights reserved.
5/8
Oct. 2013 - Rev.A
BM2P0XX series PWM Flyback converter Technical Design
Application Note
1-2-5. VCC capacitor: C2
A VCC capacitor is needed to stabilize the IC’s VCC voltage.
Capacitance of 2.2μF or above is recommended (example: 50V,
10μF).
Next, determine the startup time of the IC at power-on.
Figure 1-5 illustrates VCC capacitor capacitance and startup time
characteristics.
Figure 1-5. Startup Time (Reference Values)
1-2-6. VCC winding surge-voltage limiting resistor: R2
Based on the transformer’s leakage inductance (Lleak), a large surge-voltage (spike noise) may occur during the instant
when the MOSFET is switched from ON to OFF. This surge-voltage is induced in the VCC winding, and as the VCC voltage
increases the IC’s VCC overvoltage protection may be triggered.
A limiting resistor R2 (approximately 5Ω to 22Ω) is inserted to reduce the surge-voltage that is induced in the VCC winding.
Confirm the rise in VCC voltage while the resistor is assembled in the product.
1-2-7. Snubber circuits: C3, D3, R3
Based on the transformer’s leakage inductance (Lleak), a large surge-voltage (spike noise) may occur during the instant
when the MOSFET is switched from ON to OFF. This surge-voltage is applied between the MOSFET’s Drain and Source, so
in the worst case damage to MOSFET might occur. RCD snubber circuits are recommended to suppress this surge-voltage.
(1) Determination of clamp voltage (Vclamp) and clamp ripple-voltage (Vripple)
Consider to take a design-margin based on the MOSFET’s withstand voltage, when determining the clamp voltage.
Vclamp = 650V × 0.8 = 520V
The clamp ripple-voltage (Vripple) is about 50V.
(2) Determination of R3
R3  2  Vclamp 
Vclamp - VOR
Lleak  Ip 2  fsw(max)
When Lleak = Lp × 10% = 683μH × 10% = 68μH, R3 is derived as:
R3  2  520V 
520V - 65V
 145k Ω　 100k
Ω
68uH  0.81 2  70kHz
R3 loss P_R3 is expressed as
P_R3 
Vclamp
- VIN
R3
2

520
- 265V  1.41
100k Ω
2
 0.22W
A 1W component is determined with consideration for design margin.
(3) Determination of C3
Vclamp
C3 
Vripple  fsw(min)
 R3

520V
 1733pF  2200pF
50V  60kHz  100k Ω
The voltage applied to C3 is 520V – 264×1.41 = 148V.
300V or above is set with consideration for design margin.
(4) Determination of D3
Choose a fast recovery diode as the diode, with a withstanding voltage that is at or above the MOSFET’s Vds (max) value.
(Example: Rohm RFN1L7S: 200V, 0.8A)
The surge-voltage affects not only the transformer’s leakage inductance but also the PCB substrate’s pattern.
Confirm the Vds voltage while assembled in the product, and adjust the snubber circuit as necessary.
www.rohm.com
© 2013 ROHM Co., Ltd. All rights reserved.
6/8
Oct. 2013 - Rev.A
BM2P0XX series PWM Flyback converter Technical Design
Application Note
1-2-8. Output rectification diode: D4
Choose a high-speed diode (Schottky barrier diode or fast recovery diode) as the output rectification diode.
Reverse voltage applied to output diode is
Vdr  Vout(max)＋VINmax
Ns
Np
When Vout (max) = 12 V + 5% = 12.6V:
Vdr  12.6V＋372V 
12
 87V　60
A 87.4V/0.7 = 125V  200V component is determined with consideration for design margin.
Also, diode loss (approximate value) becomes Pd = Vf × Iout = 1V × 1A = 1W.
(Example: Rohm RF301B2S：200V 3A , CPD package)
Use of a voltage margin of 70% or less and current of 50% or less is recommended.
Check temperature rise while assembled in the product. When necessary, reconsider the component and use a heat sink or
similar to dissipate the heat.
1-2-9. Output capacitors: C5
Determine the output capacitors based on the output load‘s allowable peak-to-peak ripple voltage (ΔVpp) and ripple-current.
When the MOSFET is ON, the output diode is OFF. At that time, current is supplied to the load from the output capacitors.
When the MOSFET is OFF, the output diode is ON. At that time, the output capacitors are charged and a load current is also
supplied.
When ΔVpp = 200mV,
ΔVpp
0.2V
Z_C5＜

 0.05 Ω　at　60kHz (fsw　min)
Ispk
4.04A
With an ordinary switching power supply electrolytic-capacitor (low-impedance component), impedance is rated at 100kHz,
so it is converted to 100kHz.
Z_C5 ＜ 0.05 Ω　
60
 0.03 Ω　at 100kHz
100
Ripple-current Is (rms):
1 - 0.406
1 - Duty
 4.04A   1.798A
3
3
The capacitor’s withstanding voltage should be set to about twice the output voltage.
Is(rms)  Ispk 
Vout × 2 = 12V × 2 = 24V  25V or above
Select an electrolytic capacitor that is suitable for these conditions.
(Example: low impedance type 35V, 1000 μF for switching power supply )
(*) Use the actual equipment to confirm the actual ripple-voltage and ripple-current.
1-3. EMI countermeasures
Confirm the following with regard to EMI countermeasures.
(*) Constants are reference values. Need to be adjusted based on noise effects.
-
Addition of filter to input block
- Addition of capacitor between primary-side and secondary-side (C7: approximately Y-Cap 2200pF)
- Addition of capacitor between MOSFET’s drain and source (C8: approximately 1kV, 10 to 100pF)
(When a capacitor has been added between the drain and source, loss is increased. Check for temperature rise and
adjust accordingly)
- Addition of RC snubber to diode (C9: 500V 1000pF, R10: approximately 10Ω, 1W)
www.rohm.com
© 2013 ROHM Co., Ltd. All rights reserved.
7/8
Oct. 2013 - Rev.A
BM2P0XX series PWM Flyback converter Technical Design
Application Note
1-4. Output noise countermeasures
As an output noise countermeasure, add an LC filter
(L:10μH, C10: approximately 10μF to 100μF) to the output.
(*) Constants are reference values. Need to be adjusted based on
noise effects.
Figure 1-6. LC Filter Circuit
1-5. Proposed PCB layout
A proposed layout (example) for these circuits is shown in Figure 1-7.
・ Single-sided board, lead component view
・ Components in red are surface-mounted components
Figure 1-7. Proposed PCB Layout (Example)
www.rohm.com
© 2013 ROHM Co., Ltd. All rights reserved.
8/8
Oct. 2013 - Rev.A
Notice
Notes
1) The information contained herein is subject to change without notice.
2) Before you use our Products, please contact our sales representative and verify the latest specifications :
3) Although ROHM is continuously working to improve product reliability and quality, semiconductors can break down and malfunction due to various factors.
Therefore, in order to prevent personal injury or fire arising from failure, please take safety
measures such as complying with the derating characteristics, implementing redundant and
fire prevention designs, and utilizing backups and fail-safe procedures. ROHM shall have no
responsibility for any damages arising out of the use of our Poducts beyond the rating specified by
ROHM.
4) Examples of application circuits, circuit constants and any other information contained herein are
provided only to illustrate the standard usage and operations of the Products. The peripheral
conditions must be taken into account when designing circuits for mass production.
5) The technical information specified herein is intended only to show the typical functions of and
examples of application circuits for the Products. ROHM does not grant you, explicitly or implicitly,
any license to use or exercise intellectual property or other rights held by ROHM or any other
parties. ROHM shall have no responsibility whatsoever for any dispute arising out of the use of
such technical information.
6) The Products are intended for use in general electronic equipment (i.e. AV/OA devices, communication, consumer systems, gaming/entertainment sets) as well as the applications indicated in
this document.
7) The Products specified in this document are not designed to be radiation tolerant.
8) For use of our Products in applications requiring a high degree of reliability (as exemplified
below), please contact and consult with a ROHM representative : transportation equipment (i.e.
cars, ships, trains), primary communication equipment, traffic lights, fire/crime prevention, safety
equipment, medical systems, servers, solar cells, and power transmission systems.
9) Do not use our Products in applications requiring extremely high reliability, such as aerospace
equipment, nuclear power control systems, and submarine repeaters.
10) ROHM shall have no responsibility for any damages or injury arising from non-compliance with
the recommended usage conditions and specifications contained herein.
11) ROHM has used reasonable care to ensur the accuracy of the information contained in this
document. However, ROHM does not warrants that such information is error-free, and ROHM
shall have no responsibility for any damages arising from any inaccuracy or misprint of such
information.
12) Please use the Products in accordance with any applicable environmental laws and regulations,
such as the RoHS Directive. For more details, including RoHS compatibility, please contact a
ROHM sales office. ROHM shall have no responsibility for any damages or losses resulting
non-compliance with any applicable laws or regulations.
13) When providing our Products and technologies contained in this document to other countries,
you must abide by the procedures and provisions stipulated in all applicable export laws and
regulations, including without limitation the US Export Administration Regulations and the Foreign
Exchange and Foreign Trade Act.
14) This document, in part or in whole, may not be reprinted or reproduced without prior consent of
ROHM.
Thank you for your accessing to ROHM product informations.
More detail product informations and catalogs are available, please contact us.
ROHM Customer Support System
http://www.rohm.com/contact/
www.rohm.com
© 2013 ROHM Co., Ltd. All rights reserved.
R1102A