In switch mode power supply applications using current sensor

Common control programmes, like the current mode control and peak current limit, in the absence of traditional AC current sensor provides real time information is not possible.

Designers often use transformers, operational amplifiers and passive discrete components to implement these sensors, although the market has a number of single-chip solution. They insisted on using discrete circuits design has a number of reasons, including cost and/or performance, while also looking forward to a better single AC current sensor programme appears. But so far, they see also only have a small amount of old technical improvements.

What factors are most important?

Cost pressures for a great power system, the designer's requirements list above all costs, so the AC current sensor installation costs must be attractive (installation cost refers to the sensor itself costs plus peripherals costs, as well as additional manufacturing costs, such as calibration, etc.).

The second is the adoption of the current channel power loss minimizing to improve efficiency of low resistance number valid serial resistance (ESR), in point of load (POL) regulator such high current devices are particularly important because each additional mohm ESR will rise up to 1% of inefficiency. In addition to the cost and efficiency, but also small in size, which is installed into the circuit board power supply module is a key requirement. Other considerations include the high precision (can simplify or eliminate system internal calibration), sufficiently high isolation voltage (AC/DC converter that was an important consideration), and is used for high frequency application wide working width.

Sensor types

Available current detection solution can be divided into two categories: that is, single-chip solution and discrete circuit scheme, as shown in table 1.

Current sense amplifiers by measuring a small value series resistance on the voltage of a representative of the current voltage signal.

Obviously, the resistance will produce power and the power consumption with current increases, and in order to limit the noise, the amplifier bandwidth typically narrow. These characteristics make the technology best suited to small current DC system and low-frequency AC system, not suitable for those high-frequency and high current switch-mode device.

Hall effect and magneto-resistive (MR) device is tested with current flowing through the inductor produces a magnetic field to work, resulting in much lower power consumption.

But these devices work bandwidth is narrow, high volume, cost and output signal, noise, offset and temperature error, which reduces the precision of the measurement.

As the name implies, the current transformer (CT) works will flow through the primary coil current reflect to secondary, subordinate through an external load resistance into voltage.

CT has been widely accepted, because they require a minimum of external components, stable, with inherent high isolation, and cheap. But large, relatively high power consumption, and sometimes require additional circuitry to core reset. Many small CT or hand-wound mechanical integrity exists, therefore, for example tap interval consistency.

Low-end FET and DCR detection circuit are detection circuit already exists on the voltage of resistance, so in fact they themselves do not pose any loss.

In the DCR test scenario, the output filter on RC circuit makes this combination circuit looks like resistance. Connect to the "virtual resistance" on the amplifier to measure current and earlier series resistance/detection amplifier scheme is the same. Similar with DCR, low-end FET detection scheme detect voltage resistance, however, is the use of low resistance RDS (ON) as the detection of resistance. Although both of these methods require more general operational amplifiers and passive devices, but at the lowest cost and minimal loss of system still in use. These programmes on the down side, install bulky, sometimes requires additional system calibration costs to address high measurement error-error sometimes as high as 40%.

　　

Table 1: comparison of related AC current sensor list.

Faced with these ambiguous technology category, designers must strictly distinguish current sensor is good or bad, and then select the best able to achieve the objectives.

Although there are enough AC current detection solutions emerge, but many design also is not optimal, the need for further optimization, at least so far.

Great new programmes

Figure 1 shows a unidirectional current sensor is a good, low cost, high efficiency, small AC current sensor, and also has many other advantages.

　　

Figure 1: block diagram of a one-way communication sensor Si85xx.

Figure 1, the sensor consists of a metal inlay pieces and packaged in a small (4x4x1mm) QFN package contains composition in Silicon.

Embedded chip and chip select coils together constitute a coupled inductor, therefore through embedded Tablet AC current sensor out of voltage is equal to the current of a derivative (that is, v = Lm di/dt). Then slice of signal processing circuit performing a finite integral operation that produces an electric current flowing through the embedded chip's real-time signal proportional to. The signal via on-chip temperature compensator and gain stage circuit further adjustments. The final result is a full scale 2V, noise, very low and warming the current signal.

This kind of confusing simple schema but can provide many traditional current measurement techniques cannot provide advantages.

For example, by using standard CMOS process technology and semiconductor packaging enables very low cost, these two technologies, the cost of the schema may be less than the cost of installing CT are competitive, but also higher reliability and lower volume and other additional advantages. At the same time also implements a low loss, this is because the embedded movie in the current channelOnly adds 1.3 m ω series resistor and the series inductance 2nH. There is an additional advantage is through the integral operation is average, reduced output noise to a minimum, thus saving external RC filter cost and space. It even can inhibit the transformer coupling design edge noise, which does not require edge blanking. Figure 2 and Figure 3 through will not filter the output to a (in the low value of sensing resistors on the use of differential probe) measured currents and CT circuit (CT, diodes and RC filter) to demonstrate the principle of low noise. In both cases, AC current sensor is almost no noise.

　　

Figure 2: Si85xx output and detection of resistance.

　　

Figure 3: Si85xx output and CT output relationship.

How to implement this new technology

Use this current sensor method is very simple.

Connect the sensor allows current to flow from the IIN IOUT-end. Reverse current (that is, to workflow from IOUT IIN's current) will result in a zero output, so it does not damage the device.

The above mentioned limited integration requirements in each current measurement cycle Integrator reset before.

Implementation of the existing gated signal connected to the reset input (R1-R4). Integrator reset criteria are simple: in the current measurement must begin immediately after reset, but before the next measurement must end. For the accuracy of the ratings, reset event should at least continue 250nS.

Slice Integrator reset logic has enough flexibility to allow such current sensor with arbitrary power system topology.

Shown in Figure 4 is used for single output Si850x reset circuit. These devices usually can be used for non-existent transformer magnetic flux balance control of relatively simple some applications (e.g., buck and boost circuit).

　　

Figure 4: Si850x reset logic block diagram.

As shown in Figure 4, when the input is connected to TRST VDD, Integrator reset can be affected by the R1 and R2, real-time control of the signal.

In order to meet the high-frequency or/and high duty applications, you can set the resistance through scheduled TRST RTRST connect to and to shorten the reset time. In this case, the reduction of startup by R1 and R2 triggers, the duration is determined by RTRST. At a high speed action, allows the user to adjust the sensor accuracy.

This means that these products can be applied to more complex topologies framework, for example, control or monitor the transformer magnetic flux balance very important full bridge application.

This complex reset logic (Figure 5) is shown in Figure 4 is a superset of a circuit.

　　

Figure 5: Si851x reset logic.

As shown in the figure, there are three reduction algorithm can choose, namely, XOR, XNOR or AND/OR, choose basis depending on the status and R4 input MODE.

Need to reiterate that the reset event can, at its sole discretion by reset input, or triggered by reset input, and the previously described RTRST to timing. In short, RESET1 applies to step-up, isolated and non-isolated buck, and other relatively simple topologies, application for sliding RESET2 General, and the application for the entire bridge RESET3.

Application examples

Figure 6 shows the previously mentioned for simple synchronous buck converter current sensor, when Q1 is switched on, the measurement of electric current.

Synchronization FFT (Q2) out of gated signal for Integrator reset, because you want to make sure that the reset event is not compatible with the current measuring cycles overlap.

　　

Figure 6: synchronous buck converters in Si850x.

Also note that the reset input R2 should be grounded, so when the R1 as high as will (XOR) doors (Figure 4) output trigger reset start.

Scheduled resistance for Figure 6 RTRST timing shown in the illustration of the reset event cycle (tR).

Reset signal input from the drive, the best because of the increased drive and transistor can provide an additional time delay timing margin.

But with integrated drive controller is unable to access the drive input signal, so you must use the output signal to reset the drive. In this case, the reset input is usually need a divider will drive output swing to Si85xx VDD range.

Figure 7 shows a phase modulation full bridge application, use a work in Ping-Pong modes of current sensor.

Pingpong mode enables a single current sensor instead of two CT (typically used to monitor the transformer magnetic balance). Ping-pong output mode will bridge the arm of the current signal sent to the output-side separately.

　　

Figure 7: phase full-bridge application Si851x (pingpong mode).

As shown, by measuring current in Q1 and Q4 is switched on, the flow to the OUT2, Q2 and Q3 is switched on, the flow to the OUT1.

During the current cycle phase (i.e. when the Q1 and Q2 or Q3 and Q4 connected is switched on), Integrator reset. Work frequency relatively lower full-bridge allows sufficient reset time, TRST to VDD, allows the reset time become R1-R4 State functions.

Extended range of full scale

Many applications require greater than the full scale ranges 20A, which you can use a simple circuit board layout techniques to achieve (Figure 8).

　

On the left is installed in the Board's current sensor "x-ray chart". This is a standard method of installation, there is a current-carrying conductors, in the gap through the current sensor metal inlay sheet bridge, allowing all the measured currents flow through the embedded movie. The right to increase an embedded chip parallel small current bypass line, which constitute a splitter, bypass line of width and thickness are determined the split ratio. For example, a line of bypass 1mm width can be diverted from the embedded chip, so that sufficient current full scale increase of Si85xx 1.8 times, achieve 36A.

　　

Figure 8: use current bypass line to extend full scale range.

A brief summary of this article

AC current sensor used in switch mode power supply applications.

Traditional AC current sensor design tradeoff focus around select "most simple design method." However, this article describes AC current sensor to ingenious method to apply a basic technology, the ultimate form of sensor performance designers ' expectations exceeded. It has many important benefits: price, loss low, small size, wide-bandwidth, high precision, also improves system integration (in particular in the entire bridge applications), and low noise, high flexibility, can be applied to the switch to 1.2MHz 50kHz mode system. It is the 21st century power application best AC current sensor solutions ... It also is the most common current sensor!