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AN-CM-290 AC-DC Converter for Low Power Applications

Contents

Terms and Definitions

Off-line regulator An electronic voltage regulator that is designed to directly accept electric power from an AC current source
CCSS Capacitor-Coupled Switched Shunt Converter
ACMP Analog Comparator
DFF D-type Flip-Flop

References

For related documents and software, please visit:

https://www.dialog-semiconductor.com/configurable-mixed-signal.

Download our free GreenPAK Designer software [1] to open the .gp files [2] and view the proposed circuit design. Use the GreenPAK development tools [3] to modify the design into your own customized IC in a matter of minutes. Dialog Semiconductor provides a complete library of application notes [4] featuring design examples as well as explanations of features and blocks within the Dialog IC.

  1. GreenPAK Designer Software, Software Download and User Guide, Dialog Semiconductor
  2. AN-CM-290 AC-DC Converter for Low Power Applications.gp, GreenPAK Design File, Dialog Semiconductor
  3. GreenPAK Development Tools, GreenPAK Development Tools Webpage, Dialog Semiconductor
  4. GreenPAK Application Notes, GreenPAK Application Notes Webpage, Dialog Semiconductor
  5. SLG46110, Datasheet, Dialog Semiconductor
  6. AN-H65, Synchronous CCSS Regulator, Application Note, Supertex Inc.

Author: Gino Castillo

Introduction

Modern devices have driven the need for compact, low-cost off-line regulators. Off-line regulators which use inductors are efficient but are often large and costly. This application note will describe how to implement a circuit that instead uses a capacitor-coupled switched shunt regulator controlled by a Dialog GreenPAK SLG46110. This circuit can provide a low-cost AC-DC converter for low-power applications such as smart lighting.

Operation Principle of CCSS Topology

At a basic level shunt regulators consist of two elements: a voltage regulator in parallel with the load (shunt) and a current-limiting element in series between the supply and load. The shunt regulator used in this application note is specifically a capacitor-coupled switched shunt (CCSS) regulator (Figure 1). When the switch is closed, it short circuits the input current to ground. When it is open, diode D5 diverts the input current to the load. Besides the series capacitor (Cs), the highest voltage seen by the other components is one diode drop above Vout.

P58#yIS1
Figure 1: Capacitor-Coupled Switched Shunt Regulator

As with all shunt regulators, input current to a CCSS regulator is constant regardless of load but varies with input voltage and the series capacitance. Although current will always be drawn even under no-load conditions this current is mainly reactive with a small real. Input current can be estimated with the following equation:

Output voltage regulation is achieved by controlling the duty cycle of the switched shunt. The MOSFET shunt turns off when the Vout is below the desired regulation threshold, sending all the input current to the output. When Vout exceeds that threshold, the MOSFET shunt turns on, sending all the input current instead to ground and back to the input. The shunt is synchronized to turn on when the voltage across it (Vrac) is low to minimize the applied voltage step across Cs resulting in a more efficient operation.

The following diagram shows the operation of this control manner from a timing perspective.

P70#yIS1
Figure 2: CCSS Regulator Timing

Figure 2 describes the CCSS timing diagram:

Output voltage decays under load until

It hits the Vout threshold which

Turns off the shunt

Freeing the Vrac from GND

Vrac is clamped by D5 (Vout-0.6V) when Vout starts to rise until

Vrac falls below Vout as AC input

Vrac falls to Vrac threshold …

The shunt is turned on, Vrac is clamped by GND …

Output voltage decays under load and the cycle repeats.

The MOSFET cannot turn-on immediately when Vout exceeds the threshold, which results in overshoot at the output. A larger capacitance for Cout or operating the regulator over a narrower input voltage range can minimize the overshoot.

Circuit Schematic and Layout

Figure 3 depicts the circuit schematic of the low power AC-DC converter module. It uses the SLG46110 (U1) to control the CCSS. The module operates at an input AC voltage range from 90V to 260V(CN1), and over a non-isolated output of 3.3V (CN2). An optional LDO (U2) is added after the Vout to further stabilize the output voltage. The SLG46110 device generates the control signal that switches the MOSFET shunt (Q1) based on the Vout and Vrac threshold levels. 1N5817 Schottky diodes were used for D1 and D2. Since the dissipation factor (DF) of CS has a large effect on the efficiency of the circuit, a 1µF capacitor with a relatively small DF of 40 x 10-4 is used for CS. Figure 4 shows a picture of the PCB Layout of the board. Figure 5 shows a photograph of the complete design PCB described in this app note.

P87#yIS1
Figure 3: CCSS Regulator Schematic
P90#yIS1
Figure 4: CCSS Regulator PCB Layout
P92#yIS1
Figure 5: CCSS Regulator Prototype

GreenPAK Design

The project design developed in GreenPAK Designer is shown in Figure 6.

P96#yIS1
Figure 6: SLG46110 GreenPAK Design

Comparator Configuration

As shown on Figure 3 VDD of the SLG46110 device is connected to Vout and PIN6 to Vrac. ACMP0 is used to sense Vout voltage. With the additional logic, it will turn off the MOSFET shunt when Vout < 3.6V. ACMP1 is used to sense Vrac voltage. It will turn on the MOSFET shunt when Vrac < 1V. Refer to Figure 7 for the ACMPs settings.

P100#yIS1 P100#yIS2
Figure 7: ACMP0 and ACMP1 Configuration Settings

Measurements

The module is tested at the 150V AC RMS input voltage. The gate voltage of the MOSFET shunt and the output voltage signal is measured. Figure 8 displays their waveforms. The ripple of the output voltage is about 440mV.

P105#yIS1
Figure 8: Waveforms of CCSS

Below in Table 1 are the input and output measurements. The input RMS power is calculated with an AC power meter and the output power is calculated by multiplying the output voltage by the squared value of the resistor. As stated previously, Cs is a 1uF capacitor with a DF of 40 x 10-4.

Table 1: Input and Output Measurements
Input Voltage
(RMS V)
Input Current
(RMS mA)
Input Power
(RMS mW)
Output Voltage
(V)
Resistor
(Ω)
Output Power
(mW)
90
35.9
39
3.80
383
37.70
100
38.3
45
3.78
383
37.31
110
41.3
40
3.79
383
37.50
120
44.2
50
3.83
383
38.30
130
46.8
48
3.78
383
37.31
140
49.6
45
3.77
383
37.11
150
53.2
49
4.04
383
42.62
160
56.3
52
4.04
383
42.62
170
59.2
54
4.07
383
43.25
180
62.1
56
4.16
383
45.18
190
65.1
52
4.13
383
44.53
200
67.9
60
4.12
383
44.32
210
70.9
58
4.08
383
43.46
220
73.9
63
4.16
383
45.18
230
76.9
52
4.14
383
44.75
240
80.1
62
4.30
383
48.28
250
83.1
73
4.42
383
51.01
260
86.2
60
4.45
383
51.70

Conclusion

This CCSS converter provides a compact, low-cost inductorless alternative to a typical AC-DC converter system, which is ideal for low power applications. The GreenPAK has additional logic and GPIOs available which gives the flexibility to include additional functions to its switching control without buying another device. The switching circuit can be implemented in any GreenPAK with two ACMPs, so a different GreenPAK with the desired functionality could replace the SLG46110. For instance, an SLG46140 could be used instead to provide PWM control to an LED light.