Coupled-Choke Topology Improves 2-Phase Buck Converter
Power supplies for today's high-performance microprocessors require high-current, low-voltage dc-dc converters with fast transient responses. Such supplies must deliver currents of greater than 100 A at voltages of 1 V and below. Additionally, they must respond to load-current changes in nanoseconds and during load changes, and the supply's output voltage must remain within a narrow regulation boundary. However, a small amount of output-voltage droop is permitted so that the output voltage decreases within the regulation boundary when load current increases.
Usually, synchronous buck converters power microprocessors. Such converters typically step-down a 12-V input coming from a bus converter to 1 V and below. However, buck converters require higher steady-state levels and fast transient response to load changes. To achieve this, a small inductor allows quick current ramping to minimize the output capacitor requirement. Nevertheless, small inductor values lead to large ripple current in the inductor and higher switching losses for the converter.
Interleaved multiphase converters avoid this problem, because they substantially cancel the ripple current in the output capacitor. This allows the converters to reduce the output capacitor requirement for the same amount of ripple voltage, or they can reduce the inductance per phase so that the power supply can respond quicker to load current changes. As the inductance per phase in the uncoupled multiphase buck converter is reduced, phase ripple current increases and, in turn, switching losses and copper losses also increase.
An alternative approach using a coupled-choke topology in the multiphase converter eliminates the increased losses by reducing the phase ripple for the same output ripple voltage. Additionally, if a coupled choke with lower leakage inductance is used, transient response can be improved.
Many industry-standard multiphase buck controllers and converters are available today from companies such as Intersil, International Rectifier, Maxim Integrated Products and others. To compare the performance of the coupled-choke versus the uncoupled-choke topologies in a multiphase converter, we'll use the MAX8686 controller from Maxim. Two of the controllers will be used to form a 2-phase buck converter.
The MAX8686 is a current-mode, synchronous PWM step-down regulator with integrated MOSFETs. The controller operates from a 4-V to 20-V input supply and provides an adjustable output voltage from 0.7 V to 5.5 V while delivering up to 25 A per phase. The controller can be configured for single-phase as well as multiphase operation. For multiphase operation, the- MAX8686 can be configured in both master and slave mode.
The circuit in Fig. 1 illustrates the two approaches used: 2-phase converters with coupled-choke and uncoupled-choke topologies. Hence, L OUT_winding 1 and LOUT_winding 2 will be two windings of the coupled choke or can be two physically separate inductors. With coupled chokes, know whether the two windings are connected in-phase or out-of-phase.
A prototype board using the- MAX8686 is shown in Fig. 2. The converter is running at 400 kHz, while input voltage is 12 V and output voltage is 1.2 V, with a maximum-rated current of 50 A. This converter can deliver up to 40 A at 70°C with as little as 200 LFM of airflow. The LX pin is the output of the MAX8686. Fig. 3 shows inductor current waveforms along with LX voltage waveforms using two separate inductors. The two separate inductors used are Vishay 0.56-µH IHLP-4040DZ-11.
Inductor currents are combined in the output capacitors. Figs. 3b and 3c show the same waveforms for the converter using a coupled choke with two windings. The coupled choke used for this example is the BI Technologies HM00-07559LFTR. It has a self-inductance of 0.6 µH typical and minimum leakage inductance of 0.3 µH. The waveform in Fig. 3b shows inductor current when coupled-choke windings are connected out-of-phase. Fig. 3c shows current waveforms when windings are connected in-phase. The in-phase connection is not recommended, because it increases the phase current, which lowers the converter's efficiency.
From Fig. 3a, note that with two separate inductors you get only one current pulse per phase through each inductor. From Figs. 3b and 3c, we get two current pulses per switching cycle with the coupled choke. However, the in-phase connection of the winding causes current to decrease rather than increase when second phase turns on. With the coupled choke, when the windings are connected out-of-phase, ripple current cancellation occurs. Hence, with two separate inductors, it does not matter how they are connected, because there is no mutual inductance between them. The waveform in Fig. 3d shows phase current with a coupled choke whose windings are connected out-of-phase at a load current of 40 A.
The selection of the output inductor is very important for efficiency and transient response optimization. Its value is determined based on the amount of inductor ripple current. A larger inductance can reduce ripple current and increase efficiency, provided the DCR of the choke is not increased. However, this will increase the inductor's size.
Another negative effect of a larger inductor is a slow down of the output inductor current slew rate during a load transient. LIR is defined as the ratio of ripple current to load current per phase, and a compromised value of LIR is 0.2 to 0.5. LIR can be higher when more phases are used to take advantage of ripple current cancellation. The selected inductor should have a low dc resistance, and saturation current should be greater than the peak inductor current. If the inductor's DCR is used to sense output current, then the current sense signal should have sufficient amplitude for current-mode operation of the MAX8686 IC. A signal level of 10 mV minimum is recommended to avoid any sensitivity to noise.
An input capacitor is used to reduce the peak current drawn from the dc input source and to reduce noise and ripple voltage caused by the circuit switching. The input capacitor must meet the ripple current requirement imposed by the switching current. Low ESR aluminum electrolytic, polymer or ceramic capacitors should be used to avoid large voltage transients at the input during a large-step load change at the output. The ripple current specifications provided by the manufacturer should be carefully reviewed for temperature derating — a 10°C to 20°C rise in temperature is acceptable. Additional small value, low equivalent series inductance (ESL) ceramic capacitors can be used in parallel to reduce any high-frequency ringing.
The key selection parameters for output capacitors are the actual capacitance value, the equivalent series resistance (ESR), the ESL and the voltage rating requirement. These parameters affect overall stability, output voltage ripple and transient response. The output ripple voltage has three components — variations in the charge stored in the output capacitor, the voltage drop across ESR and ESL due to current flowing into and out of the capacitors. The design equations used are given in Fig. 7.
Figs. 4a and 4b show the transient load comparison for the converters with coupled choke and two separate inductors. The waveforms clearly show that the coupled-choke approach delivers a considerably improved transient response. This is because the transient load in the coupled choke is restricted by leakage inductance only and not by self-inductance. Thus, the transient response is improved with the coupled choke without decreasing phase inductance.
The waveforms in Figs. 5a and 5b show the output ripple voltage at full load using both approaches. The curves in Fig. 6 compare the efficiency for uncoupled and coupled versions of 2-phase converters. Efficiency also is improved with coupled choke. No-load current will be more with the coupled choke, and that is why the coupled choke approach will have a lower efficiency during light load conditions. At higher loads, the coupled-choke topology delivers better efficiency.