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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
Product Package MOSFET Max Output Power *1
85-265Vac
Function
RDS(ON) (max) IDP(max) Brownout VCC OVP
BM2P051F
SOP8
5.5Ω 2.6A 8W
Yes Latch stop
BM2P052F Auto restart
BM2P053F - Latch stop
BM2P054F Auto restart
BM2P091F
12Ω 1.3A 5W
Yes Latch stop
BM2P092F Auto restart
BM2P093F - Latch stop
BM2P094F Auto restart
BM2P011
DIP7
2.0Ω 10.4A 20W
Yes Latch stop
BM2P012 Auto restart
BM2P013 - Latch stop
BM2P014 Auto restart
BM2P031
3.6Ω 5.4A 15W
Yes Latch stop
BM2P032 Auto restart
BM2P033 - Latch stop
BM2P034 Auto restart
BM2P051
5.5Ω 2.6A 10W
Yes Latch stop
BM2P052 Auto restart
BM2P053 - Latch stop
BM2P054 Auto restart
BM2P091
12Ω 1.3A 7W
Yes Latch stop
BM2P092 Auto restart
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
supply specification.
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BM2P0XX series PWM Flyback converter Technical Design
ton
Lp
VIN
Ip
VIN
toff
ton
Np
Ns
VO
toff
Ls
VO
Ip
Ns
Np
Is
Lp Ls
Ip
ON
OFF
VIN Np Ns
Lp Ls Is
OFF
ON
VIN Np Ns
1. Design Example of Isolated Type Flyback Converter DCM (Discontinuous Conduction Mode)
Basic operation of flyback converter
(1) When switching is turned ON (2) When switching is turned OFF
Figure 1-1.Isolated Type Flyback Converter Circuit Example
When MOSFET is ON, current Ip flows through the
transformer’s primary-side winding Lp, and energy is
accumulated.
At that time, the diode is off.
When MOSFET is OFF, the accumulated energy is
output from the secondary-wide winding Ls, current Is
flows via the diode.
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BM2P0XX series PWM Flyback converter Technical Design
VORVIN
VOR
Duty
VO
VOR
Ns
Np
VIN
toff
ton
Ns
Np
VOVOR
0.406
65V95V
65V
VORVIN(min)
VOR
Duty(max)
5
1V12V
65V
VfVout
VOR
VO
VOR
Ns
Np
27.3uH
70kHz1.2A2 0.406-11V12V
fswmaxIomax2 Duty-1VfVout
Ls
2
2
＜
4.04A
0.406-1 1.2A2
Duty(max)-1 Iomax2
Ispk
683uH527.3uH
Ns
Np
LsLp 2
2
0.81A
5
1
4.04A
Np
Ns
IspkIppk
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.
When VIN = 95V (AC 85V x 1.4 x 0.8), VOR = 65V, Vf = 1V:
(*) 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:
1-1-3. Calculation of primary-side winding inductance Lp and primary-side maximum current Ippk
1-1-4. Determination of transformer size
Based on Po = 12W, the transformer’s core size is EI22.
Output voltage Po (W) Core size 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.
Figure 1-2. MOSFET Vds
Figure 1-3. Primary-side and Secondary-side Current Waveforms
Table 1-1. Output Voltage and Transformer Core
VIN→
GND→
VOR
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BM2P0XX series PWM Flyback converter Technical Design
aboveor turns50 is Np turns49.8
0.3T37mm
0.81A683uH
BsatAe IppkLp
Np 2
Bsa
t
Ae IppkLp
Bsa
t
Ae tonVIN
Np
turns6867.5turns
s150nH/turn
683uH
Value-AL Lp
Np 2
turns55.1A0.81A68turnsIppkNpNI ･
turns14 turns13.6
5
68
Ns 5
Ns
Np
turns1717.2turns
1V12V 1V15V
turns1
VVou
t
Vf_vccVCC
NsNd ４
ｆ
1-1-5. Calculation of primary-side turn count Np
Generally, the maximum magnetic flux density B(T) for an ordinary ferrite core is 0.4T @100°C, so Bsat = 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,
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.
1-1-6. Calculation of secondary-side turn count Ns
1-1-7. Calculation of VCC turn count Nd
When VCC = 15V, Vf_vcc = 1V,
As a result, the transformer specifications are as follows.
Core Tomita 2G8-EI22/EE22 or compatible
Lp 683 uH
Np 68 turns
Ns 14 turns
Nd 17 turns
Figure 1-4. EI22 AL-value vs. NI Limit Characteristics (Tomita 2G8-EE22)
AL-Value=150nH/turns2
NI=55.1A・turns
Table 1-2. Transformer Specifications
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BM2P0XX series PWM Flyback converter Technical Design
Ω Ω 0.56 0.64
0.81A
20mV/us
65kHz
0.406
0.4V
Ippk
20mV/us
fsw
Duty
Vcs
Ippk
20mV/ustonVcs
Ippk
Vcs_limit
R1
２２
２
２
0.05W0.56
3
0.406
0.81R1
3
Duty
IppkR1IprmsP_R1(rms)
W37.056.081.0R1IppkP_R1(peak) 2
Np
Nd
VINmaxVCC(max)Vdr ＋
＋ V5.122
60
15
374V29VVdr
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.
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).
Confirm the overload protection point while the resistor is assembled in the product.
Sensing resistance loss P_R1:
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:
When VCC (max) = 29 V,
With a design-margin taken into account, 122.5V / 0.7 = 175V 200V component is selected.
(Example: ROHM’s RF05VA2S 200V, 0.5A)
Table 1-3. Input Capacitor Selection Table
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BM2P0XX series PWM Flyback converter Technical Design
fsw(max)IpLleak VOR-Vclamp
Vclamp2R3 2
Ω Ω 100k145k
70kHz0.8168uH
65V-520V
520V2R3 2
0.22W
100
k
1.41265V-520
R3 VIN-Vclamp
P_R3 22
Ω
2200pF1733pF
100k60kHz50V 520V
R3fsw(min)Vripple Vclamp
C3
Ω
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.
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
When Lleak = Lp × 10% = 683μH × 10% = 68μH, R3 is derived as:
R3 loss P_R3 is expressed as
A 1W component is determined with consideration for design margin.
(3) Determination of C3
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.
Figure 1-5. Startup Time (Reference Values)
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BM2P0XX series PWM Flyback converter Technical Design
Np
Ns
VINmaxVout(max)Vdr ＋
＋ V87
60
12
372V12.6VVdr
Ω
Δ
＜min) (fsw 60kHzat 0.05
4.04A
0.2V
Ispk
Vpp
Z_C5
Ω Ω ＜100kHzat 0.03
100
60
0.05Z_C5
1.798A
3
0.406-1
4.04A
3
Duty-1
IspkIs(rms)
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
When Vout (max) = 12 V + 5% = 12.6V:
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,
With an ordinary switching power supply electrolytic-capacitor (low-impedance component), impedance is rated at 100kHz,
so it is converted to 100kHz.
Ripple-current Is (rms):
The capacitor’s withstanding voltage should be set to about twice the output voltage.
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)
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BM2P0XX series PWM Flyback converter Technical Design
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.
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 1-6. LC Filter Circuit
Figure 1-7. Proposed PCB Layout (Example)
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Notes
The information contained herein is subject to change without notice.
Before you use our Products, please contact our sales representative
and verify the latest specifica-
tions :
Although ROHM is continuously working to improve product reliability and quality, semicon-
ductors 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.
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.
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.
The Products are intended for use in general electronic equipment (i.e. AV/OA devices, communi-
cation, consumer systems, gaming/entertainment sets) as well as the applications indicated in
this document.
The Products specified in this document are not designed to be radiation tolerant.
For use of our Products in applications requiring a high degree of reliability (as exemplified
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equipment, nuclear power control systems, and submarine repeaters.
ROHM shall have no responsibility for any damages or injury arising from non-compliance with
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