Introduction

Amongst all energy issue correction (PFC) topologies, totem-pole bridgeless PFC supplies one of the best effectivity; subsequently, it’s broadly utilized in servers and information facilities. Nevertheless, closing the present management loop of a steady conduction mode (CCM) totem-pole bridgeless PFC just isn’t as simple as it’s for a conventional PFC. A standard PFC working in CCM employs a median current-mode controller [1], as proven in Determine 1, the place V_REF is the voltage-loop reference, V_OUT is the sensed PFC output voltage, Gv is the voltage loop, V_IN is the sensed PFC enter voltage, I_REF is the current-loop reference, I_IN is the sensed PFC inductor present, G_I is present loop, and d is the responsibility ratio of pulse-width modulation (PWM). Because the bridge rectifier is utilized in a conventional PFC, all these values are optimistic, and present suggestions sign I_IN is the rectified enter present sign.

Determine 1 Common current-mode controller for PFC the place all of the parameters listed have optimistic values and I_IN is the rectified enter present sign. Supply: Texas Devices

New suggestions sign

Because the inductor present within the totem-pole bridgeless PFC is bidirectional, the current-sense methodology utilized in conventional PFC won’t work. As a substitute, you will want a bidirectional present sensor corresponding to Corridor-effect sensor to sense the bidirectional inductor present and supply a suggestions sign to the management loop.

The output of the Corridor-effect sensor won’t 100% match the sensed present, although. For instance, if the sensed present is a sine wave, then the output of the Corridor-effect sensor is a sine wave with a DC offset, as proven in Determine 2. Thus, you possibly can’t use it because the suggestions sign within the current-mode controller proven in Determine 1, and you’ll have to modify the controller to accommodate this new suggestions sign. On this energy tip, I’ll describe 3 ways to shut the present management loop with this new suggestions sign.

Determine 2 Totem-pole bridgeless PFC and its current-sense sign displaying that the Corridor-effect sensor output won’t 100% match the sensed present. Supply: Texas Devices

Technique 1: Controllers and not using a unfavourable loop reference

Some digital controllers, such because the UCD3138 from Texas Devices (TI), use a {hardware} state machine to implement the management loop; subsequently, the entire enter indicators to the state machine have to be better or equal to zero. In such circumstances, comply with these steps to shut the present management loop:

1. Sense the AC line and AC impartial voltage by way of two analog-to-digital-converters (ADCs) individually.
2. Use firmware to rectify the sensed V_AC sign, as proven in Equation 1 and Determine 3.

Determine 3 Utilizing the firmware proven in Equation 1 to rectify the sensed enter voltage V_AC. Supply: Texas Devices

3. Calculate the sinusoidal reference, V_SINE, utilizing the identical methodology as when calculating I_REF in conventional PFC, as proven in Equation 2 and Determine 4.

Determine 4 Calculating a sinusoidal reference (V_SINE) utilizing the identical methodology as when calculating I_REF in conventional PFC. Supply: Texas Devices

4. Use a Corridor-effect sensor output as the present suggestions sign I_IN immediately (Equation 3).

5. Throughout the optimistic AC cycle, should you evaluate the form of V_SINE and the Corridor-effect sensor output, they’ve the identical form. The one distinction is the DC offset. Use Equation 4 to calculate the current-loop reference, I_REF.

6. The management loop has commonplace unfavourable suggestions management. Use Equation 5 to calculate the error that goes to the management loop:

7. Throughout the unfavourable AC cycle, should you evaluate the form of V_SINE and the Corridor-effect sensor output, the distinction just isn’t solely the DC offset; their shapes are reverse as effectively. Use Equation 6 to calculate the current-loop reference, I_REF.

8. Throughout the unfavourable AC cycle, the upper the inductor present, the decrease the worth of the Corridor-effect sensor output. The management loop wants to vary from unfavourable suggestions to optimistic suggestions. Use Equation 7 to calculate the error going to the management loop.

Technique 2: A pure firmware-based controller

For a pure firmware-based digital controller such because the TI C2000 microcontroller, the management loop is applied with firmware, which implies that the interior calculation parameters may be optimistic or unfavourable. In such circumstances, comply with these steps to shut the present management loop:

1. Sense the AC line and AC impartial voltage by way of two ADCs. Then use the road voltage to subtract the impartial voltage to acquire V_IN, as proven in Equation 8 and Determine 5.

Determine 5 Calculating V_IN after utilizing the road voltage to subtract the impartial voltage_. Supply: Texas Devices

2. Calculate the sinusoidal current-loop reference, I_REF, utilizing the identical methodology as in conventional PFC, as proven in Equation 9 and Determine 6.

Determine 6 Calculating I_REF utilizing the identical methodology as the standard PFC_. Supply: Texas Devices

3. Should you evaluate the form of I_REF and the Corridor-effect sensor output, they’ve the identical form; the one distinction is the DC offset. Use Equation 10 to calculate the enter present suggestions sign, I_IN. Determine 7 exhibits the waveform.

Determine 7 The waveform of the Corridor sensor output and DC offset to calculate I_IN. Supply: Texas Devices

4. Throughout the optimistic AC cycle, the management loop has commonplace unfavourable suggestions management. Use Equation 11 to calculate the error going to the management loop:

5. Throughout the unfavourable AC cycle, the upper the inductor present, the decrease the worth of the Corridor-effect sensor output; thus, the management loop wants to vary from unfavourable suggestions to optimistic suggestions. Use Equation 12 to calculate the error going to the management loop.

Technique 3: Obligation-ratio feedforward management

Whole harmonic distortion (THD) necessities have gotten stricter, particularly in server and data-center functions. Decreasing THD necessitates pushing the control-loop bandwidth increased and better. Excessive bandwidths cut back section margins, leading to loop instability. The restricted PFC switching frequency additionally prevents bandwidths from going very excessive. To resolve this downside, you possibly can add a precalculated responsibility cycle to the management loop to generate PWM; that is referred to as duty-ratio feedforward management (d_FF) [2], [3].

For a lift topology working in CCM mode, Equation 13 calculates d_FF as:

This duty-ratio sample successfully produces a voltage throughout the change whose common over a switching cycle is the same as the rectified enter voltage. An everyday current-loop compensator adjustments the responsibility ratio round this calculated duty-ratio sample. Because the impedance of the enhance inductor on the line frequency may be very low, a small variation within the responsibility ratio produces sufficient voltage throughout the inductor to generate the required sinusoidal present waveform in order that the current-loop compensator doesn’t have to have a excessive bandwidth.

Determine 8 depicts the ensuing management scheme. Including the calculated d_FF to the standard common current-mode management output, d_I, leads to the ultimate responsibility ratio, d, used to generate the PWM waveform to regulate PFC.

Determine 8 Obligation-ratio feedforward management for PFC the place including the calculated d_FF to the standard common current-mode management output, d_I, leads to the ultimate responsibility ratio, d, used to generate the PWM waveform to regulate PFC. Supply: Texas Devices

To leverage the benefits of d_FF in a totem-pole bridgeless PFC, comply with these steps to shut the present loop:

1. Observe steps 1, 2, 3, 4 and 5 from Technique 2.
2. Calculate d_FF, as proven in Equation 14. Since V_IN is a sine wave and its worth is unfavourable in a unfavourable AC cycle, use its absolute worth for the calculation.

3. Use Equation 15 so as to add d_FF to the G_I output, d_I, and procure the ultimate d.

It’s also possible to use d_FF management for a {hardware} state machine-based controller; for particulars, see reference [2].

Closing the present loop

Closing the present loop of a totem-pole bridgeless PFC just isn’t as simple as in a conventional PFC; it could additionally differ from controller to controller. This energy tip may also help you eradicate the confusion round control-loop implementations in a totem-pole bridgeless PFC, and select the suitable methodology on your design.

Bosheng Solar is a techniques engineer in Texas Devices, specializing in growing digital managed high-performance AC/DC options for server and trade functions. Bosheng obtained an M.S. diploma from Cleveland State College in 2003, and a B.S diploma from Tsinghua College in Beijing in 1995, each in Electrical Engineering. He holds 5 US patents.

Associated Content material

• Energy Ideas #108: Present sensing issues in a bridgeless totem pole PFC
• A comparability of interleaved enhance and totem-pole PFC topologies
• Energy Ideas #116: cut back THD of a PFC
• Energy Ideas #132: A low-cost and high-accuracy e-meter answer

References

1. Dixon, Lloyd. “Excessive Energy Issue Preregulator for Off-Line Energy Provides.” Texas Devices Energy Provide Design Seminar SEM600, literature No. SLUP087, 1988.
2. Solar, Bosheng. “Obligation Ratio Feedforward Management of Digitally Managed PFC.” Energy Methods Design, Dec. 3, 2014.
3. Van de Sype, David M., Koen De Gussemé, Alex P.M. Van den Bossche, and Jan A. Melkebeek. “Obligation-Ratio Feedforward for Digitally Managed Increase PFC Converters.” Printed in IEEE Transactions on Industrial Electronics 52, no. 1 (February 2005): pp. 108-115.

The publish Energy Ideas #138: 3 methods to shut the management loop for totem-pole bridgeless PFC appeared first on EDN.

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