Ripple Current and Ripple Voltage: Fundamentals and Practices for DC-DC Converter Design

What is ripple current?

Ripple current in DC-DC converters (especially buck-type) refers to the periodic fluctuation component that is superimposed on INDUCTORS current and output current due to SWITCHES operation.
A buck converter rapidly switches on and off MOS FETS SWITCHES element, smoothing it with INDUCTORS and CAPACITORS to obtain a DC output. At this time, INDUCTORS current is not a perfect DC current, but fluctuates in a triangular wave shape around the average value. This fluctuation is the ripple current. Figure 1 shows an outline of the waveform, noting the average current, ΔIL(pp), and period.

Period: Depends on SWITCHES frequency
Amplitude: Determined by input voltage, output voltage, INDUCTORS value, and load current
Waveform: Basically a triangular wave (in CCM. In DCM, the waveform becomes asymmetrical)

When the ripple current is large, the voltage drop due to the ESR of the output CAPACITORS becomes dominant, and the output ripple voltage increases. This can cause noise to worsen and efficiency to decrease. The causes of output ripple voltage will be explained in detail later.

Figure 1 Ripple current in a Buck converter

Ripple current calculation formula

ΔI_L: Peak-to-peak value of ripple current
V_in: Input voltage
V_out: Output voltage
D: Duty ratio
L: INDUCTORS
f_sw: SWITCHES frequency

Factors that determine ripple current ΔIL

As can be seen from the above formula,

1. INDUCTORS value L

The larger L, the slower the current changes and the smaller the ripple current.
Large L → Small ΔIL
Small L → Large ΔIL (i.e., miniaturization tends to increase ripple current)

2. SWITCHES frequency fSW

The higher fSW, the shorter the on time becomes, and INDUCTORS current becomes less likely to change, resulting in a smaller ripple current.
High frequency → small ΔIL
Low frequency → Large ΔIL

3. Relationship between input voltage Vin and output voltage Vout

In a Buck converter, the voltage applied to INDUCTORS when SWITCHES is turned on is
V _L = V _in − V _out
The larger this voltage difference, the greater the rate of current change and the larger the ripple current.

Disadvantages of large ripple current

1. The output ripple voltage increases

The larger INDUCTORS current ripple, the larger the current that flows through the output CAPACITORS, and the output voltage ripple increases due to CAPACITORS 's ESR. This leads to increased output voltage fluctuations and reduced quality, such as increased noise and reduced POWER SUPPLIES accuracy.

2. Current stress on the output CAPACITORS

If the ripple current flowing through CAPACITORS increases, heat generation due to ESR increases, shortening CAPACITORS 's lifespan. Also, with small CAPACITORS, the allowable ripple current may be exceeded.

3. Increased heat generation in INDUCTORS

If the ripple is large, the AC magnetization of INDUCTORS increases, which increases copper loss and iron loss.* This increases the risk of temperature rise, efficiency decrease, and saturation.

• Copper loss: Loss proportional to the load (square of the current). Occurs in the coil.
• Iron loss: A constant loss (no-load loss) regardless of the load. Occurs in the iron core.

4. Increase in EMI

The larger the rate of change of current (di/dt), the worse the EMI tends to be, often requiring larger FILTERS or additional countermeasures.

On the other hand, advantages when ripple current is large

1. INDUCTORS can be made smaller

To reduce ripple, INDUCTORS L must be made larger, but conversely, if a certain amount of ripple is tolerated, INDUCTORS size can be made smaller. Making the inductor smaller reduces costs and reduces the height of the component, which is particularly advantageous for small POWER SUPPLIES.

2. Improved responsiveness

Reducing L makes the current slope steeper, making it easier to improve responsiveness to load fluctuations.

Ripple voltage generation

The output ripple voltage Vripple is generally the sum of the following three components, as outlined in Figure 2.

ESR component: This is the main component of ripple voltage, and even an ESR of a few mΩ has a large effect. It is dominant in low to mid-range frequency components.
Capacitive component: Component due to triangular wave with fundamental period
ESL component: A component that is dominant in high frequency components. The main cause of spike noise

Figure 2 Ripple voltage generated by ESR, ESL, and C

Calculation example of ripple current and ripple voltage

As an example, let's assume a Buck converter and perform calculations under the following conditions.
Vin = 12 V, Vout = 5 V, Iout = 1 A, fSW = 1 MHz, L = 10 µH

where:

Therefore, the ripple current ΔIL is

Generally, POWER SUPPLIES IC manufacturers recommend a ripple current of approximately 20 to 40% of the rated load current.
The above result shows that the ripple current is 0.29A for an output current of 1A, which is about 30%, which is generally a good figure.

Next, calculate the output ripple voltage using the following formula (ESL components are omitted):

For the calculation, information on the capacitance value and ESR of the output CAPACITORS is required.
Here, the calculation is performed under the following conditions:
C=22µF, ESR=10mΩ

① Output ripple voltage (ESR component)

② Output ripple voltage (capacitive component)

③ Total ripple voltage

In reality, we check whether the required ripple voltage is within the specifications, but 4.6 mV pp is a relatively good value.
If you want to lower the output ripple even more:
Increase C (e.g., 22 µF → 47 µF → 100 µF)
Use CAPACITORS with even lower ESR (e.g., 2-5 mΩ polymer type)
Such measures are effective.

Summary

In a DC-DC (Buck) converter, SWITCHES operation causes INDUCTORS current to fluctuate in a triangular wave pattern around the average value, and this fluctuation is called ripple current. If the ripple current is large, the current flowing to the output CAPACITORS also increases, causing periodic fluctuations in the output voltage (ripple voltage). While this has disadvantages such as noise, heat generation, and EMI deterioration, it also has the advantage of allowing INDUCTORS to be made smaller and improving responsiveness.
In the example calculation of ripple current and voltage, calculations were performed under the conditions of Vin = 12 V, Vout = 5 V, Iout = 1 A, fSW = 1 MHz, and L = 10 uH, and when C = 22 µF and ESR = 10 mΩ, the output ripple voltage was a good result of 4.6 mV.