How to scale a power supply for voltage drop compensation

Carsten Thiele, senior field applications engineer for analog at Texas Instruments, explains how to scale a power supply for voltage drop compensation.

Wouldn’t it be nice if existing power supply designs could be easily extended to compensate for known cable and switch voltage drops under load conditions? This article will present a solution that works for almost any power supply design where a feedback voltage divider is available. To aid in the design, various calculations are set and presented in the form of an application note. This article is based on the specific example of a car center console USB charging port, which is primarily powered by an electronic device located somewhere on the dashboard. To charge mobile digital devices, the USB current capability must reach 2A or more. However, the stringent USB port supply voltage limitations are often in direct conflict with the use of low-cost thin cables, which must overcome the huge voltage drop.

Voltage drop on power cord and connector

Figure 1: True equivalent circuit diagram

Based on the true equivalent circuit diagram given in Figure 1, we can see the voltage drop in the system. The voltage Vload depends on the current Iload, the circuit resistance Rwire and the connector resistance Rcon. Basically, everything in series with the power line, such as additional switches, must be considered. Vload will decrease accordingly. Figure 2 shows this characteristic.

Figure 2: Pressure drop

If Rdrop is known and fixed in the system, then the following method can be used to modify the power supply to compensate for the voltage drop and keep Vload constant.

Car center console USB port charger example

The need for compensation can be demonstrated using the example of a car center console USB port (Figure 3). The infotainment head unit contains various electronic components and is located in the car dashboard. The USB port is a passive implementation located in the center console connected via a 3m cable. To reduce cost and weight, the wire diameter or cross-section of the cable needs to be minimized.

Figure 3: Block diagram of a car center console USB charging port

Figure 4 is the equivalent circuit for a USB charging port in the center console of a car. The power supply maintains a constant Vout voltage as the feedback resistor divider can be referenced here. To ensure that the proper charge is passed, a USB charging port controller and power switch are required. The TPS2546-Q1 (http://www.ti.com/product/tps2546-q1) provides an electrical signature on the D+/D lines to support a variety of charging schemes. The switch is placed in series with the power line to focus on the impedance added by the switch, connector, and wires for unnecessary voltage drops.

Figure 4: Equivalent circuit of a car center console USB port

USB port voltage drop problem

According to USB definition, the VBus voltage limits are 4.75V and 5.25V. To handle the voltage drop at maximum current, the output of the DC/DC converter should be set to the maximum voltage. Assuming a 2% error accuracy, the maximum allowable rated voltage of 5.19V does not exceed the maximum of 5.25V. Based on the tolerance, the minimum voltage is 5.14V. We must consider the following: the minimum Vout voltage of 5.14V minus the minimum VBus voltage of 4.75V means that 390mV is the margin we have left, which is also the acceptable overall voltage drop. Fast charging requires a current of 2.1A. Looking at the data sheet of the USB charging port controller and power switch TPS2546-Q1 and considering the worst case over the entire temperature range, we find that the RDSon of the 120m power switch will cause a further voltage drop of 252mV. After subtracting from the initial 390mV window, we get a voltage drop tolerance of 138mV for the cable and connector. Plugging the 2.1A current into Ohm's law, we find that there is 66m left for the cable and connector. Based on this and the copper resistivity that can be calculated for the wire, we can see that a 2mm2 cross section is not only large, expensive, but also heavy. Furthermore, this wire is rated for about 30A.

Implementing voltage drop compensation

As shown in Figure 5, voltage drop compensation can be implemented by measuring the current with the current shunt monitor INA213. The voltage output Vcs is fed back to the feedback resistor network of the converter through Rm. The converter block P1 illustrates the very basic principle of the control loop. The feedback FB is like a reference voltage, and Vout is regulated by the actuator scheme, where FB is VREF. Therefore, the feedback voltage FB can be considered constant and equal to the converter's VREF, which can be used as the basis for all calculations. The reverse measurement of the load current Iload through the shunt resistor Rs is the key to the application. If Vout is to increase when the load current Iload increases, the output voltage Vcs of the current shunt monitor needs to decrease. Under no-load conditions, Vcs is VREF. If the load increases, Vcs will decrease and Vout will increase accordingly. This is exactly what we want.

Figure 5: INA213 voltage drop compensation

With this implementation, the power supply characteristics will change as shown in Figure 6.

Figure 6: Voltage drop compensation

Bidirectional Current Shunt Monitor

For this application, the INA213A-Q1 (http://www.ti.com/product/ina213a-q1) from the INA21x family can be used. These are very suitable devices for implementing voltage drop compensation. It supports high-side current measurement and provides a common-mode range that extends the supply voltage range. Therefore, V+ can be directly connected to Vout. In addition, the bidirectional feature is also important to achieve the required negative polarity characteristics of the output. With the addition of zero-drift technology, these devices provide excellent accuracy and offset voltage can be as low as 35V (maximum, INA210). This can achieve a shunt voltage drop of 10mV at full flow, allowing very low resistance current shunts. Various fixed gain types are available for different load currents. Not only can the static current be as low as 100A (maximum), but it also uses small SC70 or THIN QFN packages. For the example of a car central control USB port charger, automotive grade quality requirements must be met, and this series supports this.

Component sizing and calculations

Prerequisites and operating points of voltage drop resistors

A prerequisite for properly compensating for voltage drop is to know the voltage drop resistance, Rdrop. As shown in Figure 1, Rdrop includes all resistances between the output voltage and the connection point where the voltage must remain constant. This means wires, connectors, switches, etc. Therefore, the first step is to calculate the voltage drop that needs to be compensated, which is mainly calculated using Rdrop and the maximum load current according to Ohm's law.

Feedback Divider Network

With the formula for the current sense output, the next step is to calculate the feedback divider network using the following formula.

Figure 7: Feedback voltage divider network

According to Kirchhoff's law:

Vcs1 and Vcs2 can be determined by equation 4. Please select the value of R2. Best practice is to choose a value that already exists in the power supply design, as this helps ensure network stability. The feedback voltage Vfb of the converter can be found in the regulator data sheet. From this we can solve the linear system equations manually or with mathematical tools, and the result is the conductance values ​​of G1 and GM. In turn, the result is R1 and RM.

in conclusion

If the voltage drop resistor in the system is constant, the supply line voltage drop can be compensated. The power supply can be modified with a bidirectional high-side current shunt monitor to boost Vout as Iload increases. Only minor modifications are required to an existing dedicated power supply. We can also simulate using Tina-TI, a free SPICE-based analog simulation program. Design kits and evaluation boards are also available.