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Selecting the optimal conversion frequency for dc/dc converter

higher conversion frequency has obvious advantages, but it also has disadvantages; Designers need to deeply understand the trade-offs between advantages and disadvantages and find the best (sweet spot) for the design. This practical article will also provide you with considerations for comparing the advantages and disadvantages

dc/dc converters with higher conversion frequency are becoming more and more popular because they have smaller output capacitance and inductance size to save on-board area. On the other hand, as the processing core voltage decreases (below 1V), the requirements for point of load (POL) power supply will also increase. Due to the reduction of duty cycle, it is difficult for lower voltage to achieve higher frequency

many power IC suppliers provide the market with a large number of higher speed dc/dc converters to save on-board area. A dc/dc converter with a conversion frequency of 1 or 2MHz looks ideal, but in addition to its size and efficiency, it also needs to have a deeper understanding of the impact on its power supply system. The following design examples reveal the advantages and disadvantages when a higher conversion frequency is used

select application

three different power supplies are designed and built here to demonstrate the tradeoff between high conversion frequencies. For all three designs, the input voltage is 5V, the output voltage is 1.8V, and the output current is 3a. This requirement typically comes from the power supply of high-performance processors such as DSP, ASIC or FPGA. In order to improve the design and expected performance of the filter, the allowable ripple voltage is only 20mV, about 1% of the output voltage, and the peak to peak inductance current is selected as 1a

frequency points of 350, 700 and 1600khz are selected for the three independent designs, which will be used to compare and explain the corresponding advantages and disadvantages. Tps54317 is a 1.6mhz, low-voltage, 3A Synchronous Buck dc/dc converter with integrated MOSFET (metal oxide semiconductor field effect transistor), which is selected as a voltage regulator in each example. Tps54317 from Texas Instruments has the characteristics of frequency programmable and external compensation, and is designed to be used in load point power supply applications of high-density processors

select inductance and capacitance

the inductance and capacitance values can be selected according to the following simplified equation:

equation 1:

v = L x di/dt

after the item is shifted: l ≥ Vout x (1-D)/（ Δ I x FS)

where: Δ I = 1 a (peak to peak), d = 1.8 v/5 v=0.36

equation 2:

i = C x dv/dt

after shift: C ≥ 2 x Δ I/(8 x Fs x Δ 5)

where: Δ V = 20 mV, I = 1 a peak to peak

equation 2 assumes that the capacitance used can ignore the series resistance, which is true for ceramic capacitors. Because ceramic capacitors have low resistance and small size, they are used in the above three designs. In equation 2 above, the capacitance is calculated by the product of the two terms after the term shift. The capacitance will decrease with the decrease of DC bias, but this effect is not included in most ceramic capacitance data sheets

the circuit in Figure 1 is used to evaluate the performance of the above three designs

figure 1:tps54317 reference design diagram

the next stage of the gain/signal conditioning unit is the analog filter, which will suppress the output frequency band that degrades the analog-to-digital (a/d) conversion. The next stage in series in the signal path is analog-to-digital conversion. The gain and filtered analog signal will be converted into the digital signal through ADC and transmitted to the digital processor

components not marked with values in the above schematic diagram shall be adjusted in each design. The output filter consists of L1 and C2. The component values used for the three designs are listed in the resolution of 1 second in Table 1. The selection of component values is based on the calculation results of the above equation

table 1:capacitors and inductors selected at 350khz, 700kHz and 1600 kHz respectively

it can be noted here that the DC impedance of the selected inductor decreases with the increase of frequency, because the length of copper conductor required for inductors with fewer turns is shorter. The error compensation elements of the amplifier are designed for different conversion frequencies. However, the calculation of compensation element selection does not belong to the scope of this paper

minimize on-time

the digital to digital converter integrated circuit (IC) is characterized by minimizing the controllable on-time limit, which is the narrowest pulse width that can be achieved by the pulse width modulator (PWM). In step-down converter, the percentage of field effect transistor (FET) on in the conversion cycle is called duty cycle, and its value is equal to the ratio of output voltage to input voltage

for the converter in the above example, the duty cycle is 0.36 (1.8v/5.0v), and the minimum on time of tps54317 is 150ns (maximum), as shown in the data sheet. For the limitation of controllable pulse width, the achievable minimum duty cycle is determined, which can be easily calculated by equation 3. Once the minimum duty cycle is determined, the lowest achievable output voltage can also be calculated, as shown in equation 4 and table 2

equation 3:

minimize duty cycle = minimize on time × Conversion frequency

equation 4:

minimize output voltage Vout = minimum input voltage Vin × Minimize the duty cycle (only limited to the reference voltage Vref of tps54317) table 2:minimize the output voltage under the condition of minimizing the on time of 150ns

in this example, the 1.8V output can be generated through the conversion frequency of 1.6mhz. However, if the conversion frequency is 3MHz, the possible minimum output voltage is limited to 2.3V, and the DC converter may also omit pulses. Alternative solutions include reducing the input voltage or frequency. In order to ensure that the minimum controllable on time is effective before selecting the conversion frequency, it is best to verify the data sheet of the dc/dc converter in advance

pulse omission mode

when the dc/dc converter cannot follow the door-to-door selected pulse fast enough, pulse omission will occur, so the required duty cycle cannot be maintained. Although the power supply attempts to stabilize the output voltage, further dispersion of the pulses will increase the ripple of the output voltage. When pulse omission occurs, the output ripple will exist in the secondary harmonic component, which will also lead to noise problems. At the same time, the current limiting circuit may not work properly because the IC cannot respond to large current spikes. In some cases, the control loop is not stable because the controller cannot operate completely. Minimizing controllable on-time is an important characteristic. It is advisable to verify the specifications in the data sheet of dc/dc converter to verify the combination of frequency and minimized on-time

efficiency and power consumption

dc/dc converter efficiency is one of the most important characteristics to be considered when designing power supply. Low efficiency will lead to high power consumption, which makes it necessary to add heat sinks or additional copper sheets on the printed circuit board (PCB). Power consumption also puts forward higher requirements for upstream power supply. Power consumption has the following factors:

in the above three examples, the power loss factors of interest include FET drive loss, FET conversion loss, and inductance loss. The FET impedance and IC loss in the three examples are equal, because they are designed by using the same IC that repairs and replaces defective parts or the whole machine free of charge. However, as the ceramic capacitor is selected in the example, the capacitance loss can be ignored (due to the low equivalent series resistance of the ceramic capacitor). In order to illustrate the effect of high conversion frequency, the efficiency of each of the above examples is measured and illustrated in Fig. 2

figure 2:efficiency of 5V input and 1.8V output at different frequencies

the figure above clearly shows that the efficiency decreases with the increase of conversion frequency. In order to improve the efficiency at any frequency, a dc/dc converter with low on resistance RDS (on), low gate charge or low quiescent current specifications under full load conditions should be sought, or a capacitor and resistance with low equivalent impedance should be sought

dimensions

Table 3 shows the required pad area of components with different inductance and capacitance values on the printed circuit board. Table 3: component size and overall area requirements

the recommended pad area of capacitance and inductance is slightly larger than that of independent components, which is calculated according to the above three design examples. Then, the total area is obtained by adding the respective areas of the components, including the pad area of IC, filter and other small resistors and capacitors - all of which are obtained by multiplying the component area by one or two factors. From 350khz to 1600khz, the total junction reduction is great, which can provide nearly 50% reduction of filter area and 35% reduction of on-board area, saving an area of up to 100 mm2

however, the law that the area decreases with the frequency is not unlimited, because the resistance and capacitance values cannot fall to zero! In other words, increasing the frequency will not continuously reduce the overall area. After all, mass-produced inductors and capacitors will always be limited to the appropriate size

transient response

transient response is an indicator of power performance. The following figure intercepts the bode plot of each power supply design to show the comparison with higher conversion frequency. As shown in Figure 3, the phase margin of each power supply design is between 45 and 55 degrees, indicating the well damped transient response

Figure 3: baud diagram at 350 kHz, 700 kHz and 1600 kHz

the cross over frequency is about 1/8 of the conversion frequency. When a high-speed dc/dc converter is used, it should be ensured that the error amplifier of the power supply IC has sufficient bandwidth to support high crossover frequency. Up to now, our company has been recruiting talents. The typical unit gain bandwidth of tps54317 error amplifier is 5MHz. The actual transient response time is shown in Table 4, with the associated voltage overshoot value. Table 4: transient response

due to the limitation of bandwidth, the overshoot voltage value decreases greatly with the increase of conversion frequency. The lower transient overshoot voltage is just what the new high-performance processor needs, because its voltage stabilization accuracy needs to be within 3% of the peak value of the transient voltage

when higher output current is required, Ti can provide tps40140 stackable, dual channel 1MHz dc/dc controller. The controller adopts external MOSFET, which has the advantage of interleaving multiple power supply stages and converting their output phases, so as to achieve higher conversion frequency

for example, four output terminals can be clustered (tied), and their respective conversion frequencies are 500KHz, and the effective frequency is 2MHz. Its advantages are low ripple, lower input capacitance, faster transient response and better heat dissipation management, which can spread power consumption to the whole circuit board. Through the digital bus, up to eight tps40140 can be connected and phase synchronous output can be realized, so as to maximize the effective frequency of 16mhz

summary

tradeoffs are needed in the design of high-frequency converters. Some of the advantages covered in this article include smaller size, faster transient response, and smaller voltage overshoot and undershoot. On the other hand, the main defects are the decrease of efficiency and the increase of heat dissipation

pushing de envelope has potential defects, such as pulse omission and noise problems. When selecting a dc/dc converter for high frequency applications, verify important specifications such as minimum on time, gain bandwidth of error amplifier, FET impedance, and conversion through the data sheet provided by the manufacturer

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