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5 24, 2024

With the rise of third-generation semiconductor power devices such as gallium nitride and silicon carbide in various applications, the totem-pole PFC has transitioned from academic research to real-world products. However, while benefiting from the simplicity of the topology and its high power density and efficiency, many technical challenges still need to be tackled. This article will discuss totem-pole PFC current sampling and the measured performance of the digital control totem-pole PFC controller, HP1010, recently released by Hynetek.

In traditional boost PFC circuits, inductor current detection can be as simple as a single shunt resistor. In totem-pole PFC, however, inductor current detection is not so straightforward as primarily required to address two major challenges: high voltage isolation and fast protection. As shown in Figures 1 and 2, the inductor current sampling circuit is grounded to the output voltage, hence an isolated current sampling solution is necessary. Currently, there are three main types of isolated current sampling solutions:


1.     Hall effect sensors use the hall effect to measure changes in the magnetic field, thereby measuring current. Their accuracy generally ranges from 0.5% to 5%, with low power consumption in the milliwatt range, and they support both bidirectional and DC sampling. However, weak magnetic field changes is challenging to sample, requiring signal amplification and specific medium materials to improve the performance under such incidences, which inevitably leads to size issues. In the application of Totem-Pole topology, two considerations are crucial: low latency and high power density design. The AC voltage frequency is typically 50 Hz – 60 Hz, and the sensor sampling speed must accommodate rapid response under unusual situations (such as overcurrent and lightning strikes), controlling PFC power transistor swiftly, usually in the range of several hundred kHz. Particularly for the cycle-by-cycle (CBC) overcurrent protection of primary switch, a lower transmission bandwidth can affect the rising slope of the current sampling output, preventing the controller from timely overcurrent protection. Therefore, the bandwidth is the major issue for Hall effect sensors.


2.     Isolated Current Sampling Using Differential Operational Amplifiers (OP-AMP): Differential current sampling utilizes Kirchhoff's Current Law (KCL) and Ohm's Law to measure current based on the voltage difference generated by the current passing through dimensional resistor. The direction of the current is determined by comparing the reference and the measured current. The conditioning circuit for isolated operational amplifiers is relatively complex, requiring a large area on the PCB and being susceptible to common-mode voltage interference. Additionally, the transmission bandwidth of isolated operational amplifiers is typically in the range of several hundred kHz, which also affects the effectiveness of cycle-by-cycle current protection. The cost of wider bandwidth isolated operational amplifiers is another important consideration.


3.     Current transformers(CT) samples current by inducing a current in the secondary coil from the measured current. CTs do not require a driving circuit and can measure large current with low power consumption. Compared to isolated operational amplifiers and Hall effect sensors, low-cost CTs can have relatively the highest bandwidth (up to ~MHz) and accuracy typically ranging from 0.1% to 1%, and they are less susceptible by temperature. Nevertheless, It is crucial to pay attention the issues of magnetic saturation caused by DC and decreased accuracy after the coil is magnetized. In Totem-Pole topology, the HP1010 can use three CTs to reconstruct the complete average current of the power inductor within a switching cycle, thus achieving god quality of power factor correction.


Figure 1:Totem-Pole PFC current sense:Hall(left) ; OP-AMP(right)


Figure 2:HP1010 CT Current Sensing

The HP1010 adopts the current transformer solution. This solution aims to achieve three advantages:

1. Naturally supports isolated sampling

2. Supports high-frequency  cycle-by-cycle current protection (CBC)

3. Simple and flexible implementation using Hynetek's patented circuitry

In Figure 2, the combination of CS1 and CS2 provides real-time, accurate information on the average inductor current. Additionally, CS1 integrates a high-performance analogue comparator with programmable protection thresholds, meeting the demands for dynamic overcurrent and lightning strike protection in this bridgeless operation, as confirmed by the test in Figure 3. Depending on specific design requirements, CS2 sampling can flexibly choose between resistor or CT solutions. Utilizing the sampling information, the high-performance Sigma-delta ADC of the HP1010 balances high-frequency sampling, power consumption, and average current calculation. Compared to peak current, using average current can further reduce total harmonic distortion of current (THDi) under wide load conditions, as demonstrated in the test in Figure 4.


Figure 3:HP1010 lightening test,Positive AC Cycle with Negtive Stike (L),Negtive AC Cycle with Positive Stike (R)


Figure 4:HP1010 600W Evaluation Board THDi Curve

In summary, this article mainly discusses the common current sensing schemes in Totem-Pole PFC applications. The three schemes, a) resistor with isolated operational amplifier, b) current transformer, and c) Hall effect sensor, each have their advantages and disadvantages, as listed in Table 1.


Table 1:Totem-Pole PFC Current Sense Summary

Engineers need to consider multiple factors such as system performance, device parameters, and cost when selecting a particular scheme. Test data of HP1010 confirms that the Totem-Pole topology can operate safely and reliably while achieving high efficiency. Combined with unique control algorithms, the current transformer sensing is simple, flexible, and exhibits excellent CBC protection performance. In fact, besides lightning protection, HP1010 also provides a rich and comprehensive programmable protection function, including surge overvoltage protection, input voltage overvoltage/undervoltage protection, output voltage overvoltage/undervoltage protection, and output feedback voltage open-circuit protection. The parameters enabling of protection functions can be independently configured. The I2C and UART communication interfaces ensure simple and flexible configuration and communication. Therefore, HP1010 is widely used in systems such as air conditioners, white goods, high-performance computers, 5G/telecom power supplies, industrial power supplies, ultra-high-density (UHD) power supplies, and PFC power supplies with IGBTs. For more details, visit Hynetek Semiconductor's official website: HyCtrl.

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