Three compact solutions for high step-down voltage ratios
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What you will learn:
- What are the challenges of buck converters when it comes to high reduction ratios?
- Three optional topologies to reduce steep voltage ratios.
System designers may face the challenge of down-converting high DC input voltages to very low output voltages at high output current (e.g., 60 V down to 3.3 V at 3.5 A ), while maintaining high efficiency, small form factor and simple design.
The combination of high input-to-output voltage difference with high current automatically shuts down the linear regulator due to excessive power dissipation. Therefore, the designer must opt for a switched topology under these conditions. However, even with such topologies, it is still difficult to implement a design compact enough for space-constrained applications.
This article discusses why the non-isolated DC-DC buck converter (referred to simply as buck converter in this article) faces serious challenges in down-converting high DC input voltages to very low output voltages at an output current raised. Three different approaches will be presented to lower steep voltage ratios while keeping a small form factor.
Challenges faced by DC-DC buck converters
A candidate for high down ratios is the down converter. It is the topology of choice when it is necessary to lower an input voltage to a lower output voltage (such as VIN = 12V to VOUTSIDE = 3.3 V) efficiently, with a large amount of current, while using a small footprint.
However, there are conditions where the buck converter faces serious challenges in keeping its output voltage regulated. To understand these issues, it should be remembered that the simplified duty cycle (D) of a step-down converter operating in continuous conduction mode (CCM) is:
The duty cycle also concerns the switching frequency (fSW) in the following way, where the walking time (tON) is the time the control FET remains on during each switching period (T):
Combining equation 1 and equation 2 shows how tON is influenced by the step-down voltage ratio and fSW:
Equation 3 tells us that the turn-on time decreases when there is an increase in the input-to-output voltage ratio (VIN/VOUTSIDE) and/or fSW. This means that the buck converter must be able to operate with a very low on-time to regulate the CCM output voltage under a high V.IN/VOUTSIDE ratio, and it gets even harder with high fSW.
Consider an application with VIN (MAX) = 60V, VOUTSIDE = 3.3V at IOUTPUT(MAX) = 3.5 A. If necessary, we will use the values of the LT8641 datasheet as a solution with the LT8641 will be provided in a later section. The minimum required walking time (tON (MIN)) corresponds to the highest input voltage (VIN (MAX)). To assess this tON (MIN), it is advisable to make equation 3 more precise. By including VSW(BOT) and VSW(HIGH)voltage drops for both buck converter power MOSFETs and replaces VIN with VIN (MAX)we obtain:
Using Equation 4 with VIN (MAX)FSW = 1 MHz, we obtain atON (MIN) of 61 ns. For VSW(BOT) and VSW(HIGH)we used the values provided for RDS(ON)(BOT) and RDS(ON)(HIGH) in the LT8641 datasheet, also knowing that VSW(BOT) = RDS(ON)(BOT) × IOUT(MAX) and VSW(TOP) = RDS(ON)(TOP) × IOUT(MAX).
Buck converters can rarely guaranteeON (MIN) with the short value of 61 ns obtained above; therefore, the system designer is forced to search for alternative topologies. There are three possible solutions for high buck voltage ratios.
Three compact solutions for VIN (MAX) = 60V, VOUTSIDE = 3.3V at IOUTPUT(MAX) = 3.5A
Solution 1: LT3748 Non-Optical Flyback
The first option is to use an isolated topology, where the transformer does most of the downconversion thanks to its N:1 turns ratio. Besides, Analog devices offers flyback controllers such as the LT3748 that do not require a third transformer winding or optoisolator, making the design simpler and more compact. the LT3748 solution for our conditions is presented in Figure 1.
Even though the LT3748 solution simplifies the design and saves space compared to a standard flyback design, it still requires a transformer. For applications that do not require isolation between the input and output sides, it is best to avoid this component, which adds complexity and increases the form factor compared to an unisolated solution.
Solution 2: LTM8073 and LTM4624 µModule devices
As an alternative, the designer can perform a two-step downconversion. To achieve a reduced component count of just 10, two µModule devices and eight external components can be used (Fig.2). Additionally, both µModule devices already integrate their respective power inductor, saving the system engineer a rarely simple design task. the LTM8073 and LTM4624 both come in BGA packages, with dimensions of 9×6.25×3.32mm and 6.25×6.25×5.01mm (L×W×H) respectively, providing a solution with a small form factor.
Since the LTM4624 is 89% efficient under these conditions, the LTM8073 supplies at most 1.1 A to the input of the LTM4624. Since the LTM8073 can supply up to 3 A of output current, it can be used to power other circuit rails. For this purpose, we have chosen 12 V as the intermediate voltage (VINT) in Figure 2.
Although avoiding the use of a transformer, some designers may be reluctant to implement a solution requiring two separate buck converters, especially if no intermediate voltage is required to power other rails.
Solution 3: LT8641 buck converter
Therefore, in many cases, the use of a single downconverter would be preferable as it provides the optimum solution to combine system efficiency, small footprint and design simplicity. But haven’t we just demonstrated that buck converters can’t cope with high VIN/VOUTSIDE combined with high fSW?
This statement can apply to most buck converters, but not all. For example, buck converters such as ADI’s LT8641 are specified with a very short minimum turn-on time of 35 ns typical (50 ns max) over the entire operating temperature range. These specs are well below the previously calculated minimum required turn-on time of 61 ns, providing us with a third possible compact solution. picture 3 shows the simplicity possible with the LT8641 circuit.
It should also be noted that the LT8641 solution may be the most efficient of the three. Indeed, while efficiency still needs to be optimized in relation to picture 3we can decrease fSW and select a larger inductor size.
Although fSW can also be decreased with Solution 2, the integration of power inductors does not provide the flexibility to increase efficiency beyond a certain point. In addition, the use of two consecutive downconversion stages has a slight negative impact on efficiency.
In the case of solution 1, the efficiency will be very high for a flyback design, thanks to its operation in limit mode and the fact that all components are removed with the design without optical feedback. However, efficiency cannot be fully optimized as the number of transformers to choose from is limited, unlike the range of inductors available for solution 3.
Another way to check if the LT8641 meets the requirements
In most applications, the only adjustable parameter in equation 4 is the switching frequency. Therefore, we reformulate equation 4 to evaluate the maximum allowed fSW for the LT8641 under given conditions. Doing this, we get equation 5, which is also found on page 16 of the LT8641 datasheet:
Let’s use this equation with the following example: VIN = 48V, VOUTSIDE = 3.3 V, IOUTPUT(MAX) = 1.5 A, fSW = 2MHz. A 48V input voltage is commonly found in automotive and industrial applications. By inserting these conditions into equation 5, we obtain:
Therefore, under the provided application conditions, the LT8641 would operate safely with fSW tuned as high as 2.12 MHz.
Three different methods have been presented to achieve a compact design under high step-down voltage ratios. The LT3748 flyback solution does not require a bulky opto-isolator and is recommended for designs where isolation is needed between the input and output sides.
The second method, which involves implementing the LTM8073 and LTM4624 µModule devices, is particularly attractive when the designer is hesitant to select the optimum inductor for the application and/or when an additional intermediate rail must be provided. The third method, a design based on the LT8641 buck converter, offers the most compact and simple solution when the only requirement is a large voltage drop.