Present sensing concerns in a bridgeless totem pole PFC

June 24, 2022

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Two of the highest concerns in knowledge heart and server power-supply unit (PSU) designs are energy density and effectivity. Assembly the strictest effectivity commonplace—80 Plus Titanium—is now a minimal requirement for next-generation knowledge heart and server PSUs.

The 80 Plus Titanium requires over 96% end-to-end peak effectivity, which signifies that the facility issue correction (PFC) stage effectivity have to be increased than 98.6%. The problem is that the standard PFC topology can’t meet this effectivity requirement due to bridge rectifier energy loss. Assuming a 1-V diode voltage drop and a 230-V_AC enter, a 4-kW bridge PFC topology could have 33 W of energy dissipation on the bridge rectifier and a subsequent 0.825% effectivity loss. Thus, knowledge heart designers should undertake topologies corresponding to semi-bridgeless PFC or totem-pole PFC to attain the >98.6% effectivity goal for the PFC stage.

A semi-bridgeless PFC energy stage requires two PFC inductors, and every inductor solely performs increase operation in half of an AC cycle. A bridgeless totem-pole PFC stage requires just one PFC inductor, which performs increase operation in a full AC cycle. Semi-bridgeless PFC requires a bigger footprint, whereas bridgeless totem-pole PFC permits excessive energy density and high-efficiency PSU designs. That’s why bridgeless totem-pole PFC is widespread in energy provides that adjust to 80 Plus Titanium necessities.

Determine 1 The semi-bridgeless PFC topology is proven on high and bridgeless totem-pole PFC on backside. Supply: Texas Devices

Common current-mode management is the most typical management technique for steady conduction mode (CCM) PFC, no matter topology. Common current-mode management makes use of inductor present as a substitute of instantaneous present (corresponding to peak present). In conventional bridge PFC, the PFC controller’s reference makes use of floor because the supply of a MOSFET and the unfavourable finish of an output bulk capacitor, with a current-sensing resistor between the anode of a rectified bridge and floor, as proven in Determine 2. The controller detects voltage on the sensing resistor; an inner operational amplifier converts the sign to a optimistic worth, after which sends the transformed sign to the present loop.

Determine 2 That is how present sensing is carried out in a conventional PFC circuit. Supply: Texas Devices

Implementing common current-mode management in conventional PFC is straightforward, as the present circulation is unidirectional over the current-sense resistor in addition to the PFC inductor. However as a result of the present circulation on the PFC inductor in bridgeless totem-pole PFC is bidirectional, you need to implement completely different current-sensing strategies for common current-mode management. This text will focus on three current-sensing strategies for common current-mode management bridgeless totem-pole PFC, together with their trade-offs.

Present sensing with a shunt resistor

Determine 3 is a simplified schematic of present sensing with a shunt resistor. On this circuit, the operational amplifier converts the voltage on the shunt resistor to a sign, with its peak-to-peak voltage lower than its bias provide voltage (V_CC). Including a DC bias of one-half V_CC permits the analog-to-digital converter (ADC) to learn bidirectional present indicators. The bottom reference of the management circuit is usually positioned at impartial.

Determine 3 The simplified schematic exhibits present sensing utilizing non-isolated shunt resistor technique. Supply: Texas Devices

Present sensing is usually correct, with out a important propagation delay. You may improve the bandwidth of the present loop and allow the facility stage to reply shortly to an overcurrent fault. A shunt resistor-based current-sensing technique supplies the very best sensing accuracy, so you should utilize the sensing outcomes for management and safety, in addition to for correct enter energy monitoring.

Nonetheless, shunt resistor-based bridgeless totem-pole PFC requires an advanced circuit for output voltage sensing as a result of the output floor reference and controller (MCU proven in Determine 3) floor reference usually are not on the similar node. Shunt resistor-based present sensing additionally requires extra remoted rails from the auxiliary provide and extra remoted drivers.

One other approach to allow present sensing and convert it to a management circuit is with an remoted amplifier, as proven in Determine 4. Utilizing an remoted amplifier permits the controller’s floor reference to output to floor, which can simplify the sensing and driver circuitry within the design.

Determine 4 Present sensing is carried out with a shunt resistor and remoted amplifier. Supply: Texas Devices

Among the most vital concerns for remoted amplifier-based present sensing embody:

Requires an remoted V_CC provide rail for the input-side sampling circuit.
Requires a slender enter voltage vary for low energy consumption on the shunt resistor.
Wants low nonlinearity of the output sign achieve and low error drift with temperature for sensing accuracy.
Wants a quick response to enter sign transients.

For instance, the AMC3302 remoted amplifier has an enter voltage vary of ±50 mV, enabling the choice of a shunt resistor with smaller resistance to assist scale back energy dissipation of the shunt resistor and enhance system effectivity. The AMC3302 amplifier’s output bandwidth of 340 kHz ensures quick response to the enter transient, whereas the mixing of an remoted DC/DC converter eliminates the necessity for an exterior remoted energy rail.

Present sensing with a present transformer

A present transformer is a ferrite element that requires a magnetic reset in each switching circuit. Inserting it in sequence with the primary field-effect transistor (FET) permits sampling of the present pulse, as proven in Determine 5. Theoretically, if the controller sampling level is ready at one-half the on-time (Ton), the sampled worth equals the typical present of the PFC choke in a switching cycle.

Determine 5 The schematic exhibits present sensing carried out with a present transformer. Supply: Texas Devices

In bridgeless totem-pole PFC, through the optimistic half of an AC cycle, Q2 works as the primary FET and Q1 is the synchronous FET. Throughout the unfavourable half of an AC cycle, Q1 works as the primary FET and Q2 is the synchronous FET. Sensing present at Ton on a full AC cycle thus requires the location of two present transformers in sequence with each Q1 and Q2. The controller selects a pattern worth of CT1 and CT2 from the polarity of the enter voltage.

A present transformer supplies an remoted present sign for the management circuit. If the controller floor reference is on the unfavourable finish of the output bulk capacitor, low-frequency FETs (Q3 and This fall) require solely a non-isolated driver, simplifying the sensing circuit in comparison with the shunt resistor-based sensing technique. Sadly, the nonlinear B-H curve and hysteresis loop of ferrite materials make the pattern worth inaccurate. The present distortion could be worse, particularly throughout mild hundreds or zero crossing. And along with sensing accuracy issues, having a present transformer in sequence with the primary FETs will improve the power-loop inductance, due to this fact rising voltage stresses throughout switching occasions. You could want an extra snubber to clamp the FETs’ voltage stress.

Present sensing with a Corridor-effect present sensor

Determine 6 exhibits a simplified current-sensing circuit that makes use of a Corridor-effect present sensor. The Corridor-effect present sensor outputs a Corridor potential that’s proportional to the enter present from remoted electromagnetic sign conversion. The polarity of this potential is similar as the present’s route, so the Corridor-effect present sensor is appropriate for bidirectional present sensing.

Determine 6 The schematic exhibits present sensing carried out with a Corridor-effect present sensor. Supply: Texas Devices

In comparison with the remoted amplifier technique, a Corridor-effect present sensor converts indicators by the magnetic area contained in the chip itself, eliminating the necessity for an remoted energy rail for the applying. You may design the enter conductor resistance beneath 1 mΩ for prime present sensing, reaching decrease energy loss.

A Corridor-effect present sensor’s bandwidth is within the vary of 10 kHz to 1 MHz, so for PFC with common present management, selecting a sensor with a bandwidth increased than 10 instances that of the present loop is adequate for the system. In different phrases, you want greater than 50 kHz of bandwidth within the Corridor-effect present sensor for 2- to 5-kHz current-loop PFC.

The benefits of the Corridor-effect present sensor make it choice for present sensing in bridgeless totem-pole PFC. One notable attribute of frequent Corridor-effect present sensors at present is that its wise present vary is expounded to its V_CC voltage degree. An instance is the TMCS1100A1 Corridor-effect present sensor that permits 46 A linear measurement present vary with V_CC= 5V whereas it solely permits 29 A with V_CC= 3.3V. Subsequently, it’s difficult to make use of the Corridor-effect present sensor’s full wise vary with increased V_CC whereas sending the sensor output sign to a controller with decrease V_CC.

To grasp this trick, let’s begin with Corridor-effect present sensor fundamentals. Equation 1 expresses the output of a Corridor-effect present sensor:

V_o = S × I_IN + V_Offset (1)

The place S is the sensitivity of the Corridor-effect present sensor in millivolts per amperes, I_IN is the enter present and V_Offset is the offset voltage at a zero present enter.

In most bidirectional sensors, V_Offset is normally set to one-half V_CC, so the utmost allowed vary of enter present is V_CC/2×S. A better V_CC will enable a wider wise present vary. For instance, a Corridor-effect present sensor with 3.3 V of V_CC would enable a 1.65-V/S wise present vary, whereas a Corridor-effect present sensor with 5 V of V_CC would enable a 2.5-V/S wise present vary.

Determine 7 The circuit configuration accommodates completely different values of V_CC between the Corridor-effect present sensor and the controller. Supply: Texas Devices

With a purpose to have a wider wise present vary whereas outputting to a 3.3-V most ADC, an operational amplifier can set the achieve of the sensing circuit and alter the utmost enter voltage to the ADC. Determine 7 exhibits the applying circuit. First, the Corridor-effect present sensor is sensing a wider enter present vary, with 5 V of V_CC and a pair of.5 V representing 0 A of present. Equation 2 illustrates learn how to decrease the Corridor-effect present sensor’s output sign vary based mostly on V_CC ranges (or voltage reference ranges):

1.65 V/2.5 V = R₃/R₁+R₃ (2)

You need to use Equation 3 to set the amplifier achieve in order that the utmost sensed present degree remains to be inside the allowable enter voltage vary of the controller:

I_max = 1.65 V/S × R₁/R₂ (3)

So long as you establish the utmost present degree that you just wish to sense, you’ll solely have to resolve one resistor worth (as an example, R1) and may decide the remaining (R2 and R3) utilizing Equations 2 and three.

Execs and cons

Desk 1 summarizes the professionals and cons of every current-sensing technique. Every system’s specs and necessities will allow you to decide which sensing technique to decide on.