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A Single Stage Flyback Power Supply Unit for LED lighting Applications

From: Hit:3433 Add time:2012-11-14 12:01:48

Abstract

This paper presents a 75W single-stage power supply unit for LED lighting. The proposed power supply designed using flyback converter topology that is controlled by the critical conduction mode control. The secondary side of main transformer is directly connected to LED strings. By a constant current feedback circuit, this flyback converter directly regulates LED current. Although the proposed circuit doesn’t need input current-sensing and input voltage feed-forward, it can achieve high power factor for wide input range. A prototype experimental set-up has been built and tested. Through this experiment with a prototype set-up, the validity of the proposed circuit is verified.

1. Introduction

In recent years, LED technology has been grown rapidly and it is considered as a most strong candidate for the next generation lighting source due to high energy efficiency, long life time and its positive environmental attributes. Moreover, LED technology controls the color, shape, illumination pattern as well as light itself [1]. In the case of indoor or street LED lighting application, a power supply unit(PSU) that converts AC input voltage to DC output voltage is necessary and various topologies have being considered [2-4]. The PSUs for low power LED lighting are normally single-stage converter types but two stage converters are common for a high power LED lighting applications due to their power limitation. In this paper a single-stage PSU for high power LED lighting is presented. As the power converter circuit, flyback converter topology is applied because it doesn’t need an inductive output filter, the main transformer works as an inductive filter itself and the input and output stages can be isolated [5, 6]. The proposed circuit used a critical conduction mode (CRM) PFC controller and the input voltage and switching current are not needed, the only output voltage feed-back is necessary. Through an experiment with a prototype 75W single stage flyback converter, the feasibility of the proposed single stage PSU, control scheme and feedback methods are examined and discussed in this paper.

2. Single Stage Flyback Converter for LED 

2.1 Fundamental analysis

Figure 1 shows the circuit diagram of a flyback AC-DC converter. Both CV(constant voltage) and CC(constant current) feedback circuit are needed to prevent overload condition as well as over voltage conditions. In LED lighting, the output is always full load condition and the forward voltage drop of LED will be decreased in accordance with the increasing junction temperature of LED. Therefore the output should be controlled by CC mode in the normal state while CV mode only acts as over voltage protection.


Fig. 1. Circuit diagram of a flyback AC-DC converter

A voltage mode CRM PFC controller, FAN7530 is used as a control IC and the internal block diagram is shown in Figure 2. In the control circuit, the switching signal is generated by comparing the output of the error amplifier with the internal ramp signal. Consequently, the input voltage and current are unnecessary. The turn-on time of switch is fixed while the turn-off time is varied during the steady state. Therefore, the switching frequency inevitably varies in accordance with the input voltage variation as shown in Figure 3.


Fig. 2. Block diagram of FAN7530


Fig. 3. Switching frequency variation

Figure 4 illustrates the theoretical waveforms of the primary side switch current, the secondary side diode current and gating signal. MOSFET, Q turns on and FRD, Do turns off under zero-current condition while Q turns off and Do turns on under the hard switching condition.


Fig. 4. Theoretical waveforms

In flyback converter, the transformer will be easily saturated because the transformer is only utilized within 1st quadrant. Moreover if it works under the critical conduction mode, the peak current is much higher than that of the continuous conduction mode. Therefore the air-gap should be inserted to prevent the saturation of the transformer.
When a flyback converter is applied to the single stage AC-DC converter, the proper turn ratio, N2/N1, is very important because the maximum voltage rating of MOSFET and FRD strongly relates with the turn ratio of transformer. There is a trade off relationship between the drain to source voltage rating, Vdss, of MOSFET and the reverse voltage rating, VR of FRD according to the turn ratio of the transformer. The larger turn ratio requires the higher DC reverse voltage, VR of FRD while the drain-source voltage, Vdss, of MOSFET is decreased. In contrast, the lower turn ratio causes the higher voltage stress of MOSFET while VR of FRD is decreased.

Figure 5 shows the trade-off relationship between Vdss, of MOSFET and VR of FRD. From Po=ηVinIin, the maximum input current, Iin(max) = Po/ηVin. If the switching frequency, fs is much higher than the AC line frequency, fac, the input current can be assumed to be constant during a switching period.


Fig. 5. VDS and VR in accordance with turn ratio

In order to define the magnetizing inductance of transformer, the longest period has to be defined. The longest switching period occurs at the peak point of the input current when the minimum input voltage is applied. The maximum input current and the switching peak current is defined as follow;



Where,     respectively.
The transformer primary side voltage, VT is defined as follow;

Therefore, the magnetizing inductance is obtained as follow;

The voltage stress of MOSFET is
 

Table 1. Electrical parameters
Parameters    Value
Output Power    75W
Input Voltage Range    85-265V
Output Voltage    45V
Limited output voltage    50V
Turn ratio (N2/N1)    17/44
Magnetizing inductance    297uH
Minimum switching frequency, fs_min@ Vin_min    50kHz
Efficiency, η    85 %

where Vsn is the maximum capacitor voltage of the snubber circuit, Vf (N1Vo/N2) is the flyback voltage and VLr is the ringing voltage at the leakage inductance of the transformer respectively. Normally VLr is estimated as 1.5 times of the flyback voltage. The maximum reverse voltage and the forward peak current of FRD are

respectively.

2.2 Snubber circuit design

In flyback converter, the resonant between Lleak and Coss occurs an excessively high voltage surge which causes the damage of MOSFET during the turn-off instant. This voltage surge has to be suppressed and the snubber circuit is therefore necessary to prevent MOSFET failures as shown in Figure 6.
The clamping voltage by snubber is 

Fig. 6. Snubber circuit design

The maximum power dissipation of the snubber circuit is determined by

and, the maximum power dissipation is

Where, vc=Vf +Vsn. Therefore, the resistance, Rsn is determined by

The maximum ripple voltage of snubber circuit is obtained as follow;

The larger snubber capacitor results the lower voltage ripple, but the power dissipation will be increased. Consequently, selecting the proper value is important. In general, it is reasonable to determine that the snubber voltage is 1.5 times of the flyback voltage and the ripple voltage is 50V.

3. Experimental Results

To verify the validity of the proposed circuit, a prototype 75W experimental set-up has been built and tested.

The photograph of experimental set-up is shown in Figure 7. The electrical parameters are listed in Table 1. Figure 8 shows the experimental waveforms of VGS, VDS and Id at 110Vac input and at 220 Vac input respectively, which shows the switching current waveforms follow the sharp of the input voltage well. Figure 9 shows the input voltage and current at 110 Vac input and 220 Vac input conditions. The power factors at 110 Vac and 220 Vac condition are measured as 0.997 and 0.955 respectively.

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