Power Supply IC for Low Power AC Adapters

Yasushi Katayama

1. Introduction Fig.1 Block diagram

The AC adapter is a kind of AC-to-DC converter 5 V and is mostly used for supplying power to portable VDD Regulator VIN electronics equipment or for charging batteries. Re- Reference voltage Thermal Internal power shutdown quired features of the AC adapter have been small circuit 1.18 V Logic supply circuit size, light weight and low price. Recently, however, Bias UVLO low has also come to be required due to D 50 kHz the trend toward energy saving in response to global clk environmental problems. D max D=0.5 To satisfy such requirements for the AC adapter, blk Comparator Oscillator S Q Fuji Electric developed and produced a power IC 1.18 V – FB – R () for switch-mode con- + Driver trol. The IC integrates a PWM (pulse width modula- Rs tion) control circuit containing high accuracy analog S CMOS (complementary metal oxide semiconductor) devices with a 700 V power MOSFET (metal oxide semiconductor field effect transistor). This construc- Table 1 Primary electrical characteristics tion achieves small-sized and low priced AC adapters Item Characteristic with low power dissipation and high reliability. Breakdown voltage 700 V (Minimum) This paper presents an overview of the “FA5702P” Power MOSFET power IC for switch-mode power supply control, devel- On-resistance 20 Ω (Typical) oped for application to 5 W-class low-power AC adapt- Supply voltage 500 V (Maximum) ers. VDD terminal voltage 5 V (Typical) Oscillator frequency 50 kHz (Typical) 2. Characteristics Maximum duty cycle 50 % (Typical) Control circuit Current limit 350 mA (Typical) Figure 1 shows a block diagram of the FA5702P Supply current at µ and Table 1 shows its primary electrical characteris- MOSFET switching 200 A (Typical) tics. Supply current at 60 µA (Typical) For decreasing standby power consumption of the latched off AC adapter, reduction of the power dissipation of its control circuit is a crucial issue. The FA5702P has circuit, consisting of a auxiliary wind- achieved a low supply current of 200 µA at MOSFET ing, diodes, and capacitors, becomes unnecessary switching due to significantly reduced current con- for supplying power to the IC, and thus the circuit sumption and optimized configuration of the circuit. configuration of the AC adapter can be simplified. Primary advantages of the FA5702P are as follows: (4) The IC accommodates input from any universal (1) A power MOSFET and its control circuit are AC power supply ranging from 100 V to 240 V, integrated into a single chip. since the supply voltage is 500 V maximum and (2) A low supply current of 200 µA at MOSFET the breakdown voltage of the power MOSFET is switching is achieved with a newly designed 700 V maximum. control circuit. (5) Peak current mode PWM control is utilized, which (3) The supply current for the internal circuit is fed is superior in stability and frequency response and from the rectified AC input directly. A bootstrap suppresses the generation of audible transformer

Power Supply IC for Low Power AC Adapters 9 noise under light load conditions. shown in the lower half. The IC is constructed with a (6) It contains internal protection functions such as standard DIP (dual in-line package). overcurrent limiting, thermal shutdown, and undervoltage lockout (UVLO). 3. Example of Application Circuit Figure 2 shows a top view of the FA5702P chip and Fig. 3 shows external views of the IC packages. Figure 4 illustrates an example of an AC adapter In Fig. 2, the 700 V power MOSFET is shown in circuit that utilizes the FA5702P chip. The circuit is the upper half of the figure and the control circuit is configured as a flyback converter, which is used in most low-power AC adapters, and the input range is Fig.2 Top view of FA5702P chip from 90 V to 264 V AC and the output is 5.8 V/800 mA DC . The FA5702P contains an internal power MOSFET for the main switch, and the FA5702P can be directly powered from the rectified AC input as shown in Fig. 4. Therefore, in contrast to the usual ICs for switch-mode power supply control which require the use of addition- al power MOSFETs and bootstrap and startup circuits consisting of a transformer auxiliary winding, diodes, and capacitors, the FA5702P eliminates the need for those additional components. This allows an AC adapter that uses the FA5702P to be configured with

Fig.5 Waveforms at rated output

VIN = 100 V, VOUT = 5.77 V, IOUT = 0.78 A

Fig.3 External view of FA5702P packages

VOUT : 5 V/div

OUT 0 V

DS 0 V VDS : 250 V/div D

I 0

ID : 0.2 A/div

20 µs/div

Fig.4 Example of application circuit

D1 D3 L1 T1 IOUT

R2 C4 + + C5 C6 VOUT VIN D2 VDD D + V C1 FA5702P VIN DC R1 FB PC1 R3 R4

C2 S C7 C3 IC1

VR1

10 Vol. 48 No. 1 FUJI ELECTRIC REVIEW Fig.6 Waveforms at no load Fig.8 Efficiency in application circuit

VIN = 100 V, VOUT = 5.77 V, IOUT = 0 A 100

80

VOUT : 5 V/div (%)

OUT 0 60 η V

40 Efficiency DS 0 V VDS : 250 V/div 20 Input voltage VIN = 100 V(AC)

Output voltage VOUT = 5.78 V

D 0

I 0 0 200 400 600 800 ID : 0.2 A/div Output current IOUT (mA) 20 µs/div

Fig.9 Total loss in application circuit Fig.7 Pulse skipping operation at no load

101

VIN = 100 V, VOUT = 5.77 V, IOUT = 0 A (W) 100

VDS : 100 V/div OUT P - IN DS 0 P V

–1 Pulse skipping frequency : 91.3 Hz 10 otal loss

T Standby power 38.8 mW

Input voltage VIN = 100 V(AC)

Output voltage VOUT = 5.78 V 10–2 ID : 0.04 A/div 0 200 400 600 800 D

I 0 Output current IOUT (mA)

1 ms/div

in Fig. 4. The selection of components in the trans- former and feedback circuit greatly effects the efficien- fewer components than in the case when conventional cy and total loss of the circuit, but this circuit has type ICs are used. achieved low standby power of 38.8 mW. Figure 5 shows electrical waveforms of the opera- tion of this circuit at the rated output. As can be seen, 4. Conclusion the circuit operates stably while regulating the output voltage with PWM control. This paper has introduced an overview of the Figures 6 and 7 show electrical waveforms of the FA5702P power IC for switch-mode power supply operation of this circuit under the condition of no load. control, developed for the application to 5 W-class low- In this condition, the circuit status shifts to pulse power AC adapters. skipping operation, as shown in the figures. At that This IC contains an internal power MOSFET and time, the pulse skipping frequency has decreased to an has the advantage of low current consumption. There- audible frequency level. The FA5702, however, gener- fore, it helps to reduce the standby power consumption ates no audible noise from the transformer core. This of AC adapters and the number of their circuit is because the IC controls the peak current at a low components. level with the current mode PWM control and thus the Fuji Electric will respond further to market needs variation in the magnetic flux density excited in the by expanding the product line of power ICs for switch- core is also suppressed. mode power supply control to provide devices suitable Figures 8 and 9 respectively show the efficiency for application to a variety of applications and output and the total loss measured in the application circuit requirements.

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