"Designers have been looking for suitable ready to use alternatives to provide low-voltage DC output rails in the medium intensity current range of 1 to 10 amps, while meeting basic performance objectives and efficiency and regulatory requirements. Although suppliers now provide many suitable small DC / DC Converters / regulators to meet this demand, they should not be assumed to be just "substitutes" that can be ignored.
Why do you say that? Although they do not seem complex, these regulators are still power supplies that provide medium current to the load. In terms of modules, all designers need to do is add a few external non critical passive components. However, this convenience may make designers self righteous and lead them to ignore the basic elements that affect all power supplies and their power rails.
This article will identify and discuss these important basic elements. Then, this article will introduce a variety of modular power solutions and explain how to apply these core principles to make full use of each solution.
Identify "traps" in Power Design
First, there is some good news. The operating efficiency of these equipment is relatively high, usually between 80% and 95%, depending on the specific model and operating point. Although the output current is at a medium level, designers still need to carry out basic thermal analysis and heat dissipation analysis to ensure that the device remains within the rated temperature range and does not add too much cooling burden to the system.
There are five main concerns: 1) IR pressure drop; 2) Isolation; 3) Output adjustability; 4) Switching noise; And 5) low impedance return path. First, before selecting a specific DC / DC regulator, the designer should verify that the non regulated DC power supply can provide sufficient current, and consider that other DC / DC units may also rely on the power supply. In addition, it is also necessary to ensure that the dynamic performance of the power supply is sufficient to meet the high current transient load requirements, mainly because these regulators do not have large output capacitors.
IR pressure drop: is the load too far?
When determining the location of components, I / O ports and potential heat sources, designers usually have to face various conflicting PCB layout requirements. In this regard, the power regulator may be a challenging device. Ideally, it should be placed close to the load to minimize IR voltage drop and noise pickup, while reducing the need for space wasting large-area printed circuit board routing (for passing current).
IR pressure drop is the easiest to ignore, but it is the easiest to calculate. Even if there is only a few milliohm resistance between the DC / DC regulator output and its load, it may cause the supply voltage of these units to drop by 10 MV or more. This may seem trivial, but when the nominal DC power rail is only a few volts, it may have a significant impact.
Therefore, the wiring size of the printed circuit board must be appropriate, or it can be installed on a separate printed circuit board. The use of thin busbars shall be considered. Busbars seem to be an ancient solution, but they are very effective for two reasons. First, they can significantly reduce the IR pressure drop. Secondly, the double-layer bus can be used at less additional BOM cost, so as to obtain excellent DC grounding return path.
To minimize IR pressure drop; Establish better and lower resistance system grounding; And minimize parasitic effects and non DC impedance in the grounding structure that may affect the higher frequency performance, which is as important as the DC power rail itself on the high voltage side. Of course, regardless of the physical DC power rail and grounding, it is important to make the low impedance and low value bypass capacitor as close to the IC power pin or lead as possible, and minimize the noise related problems on the power rail.
In some cases, IR voltage drop is still unacceptable, so a special regulator architecture including remote detection is required. Here, the regulator has two conventional terminals for current supply and return, and two detection terminals connected to the load for measuring the actual voltage at the load. The regulator uses this detection value as feedback to adjust its output voltage to compensate for the effect of voltage drop (Fig. 1).
For example, the ltm4601 µ module of Linear Technology Corp / analog devices ® A high current of up to 12 amps between 0.6 and 5.0 volts can be provided through a DC input of 4.5 to 20 volts. At this high current, the IR voltage drop may affect the system performance and behavior consistency. Using remote detection, the module can correct the IR voltage drop, voltage loss and ground return path of the printed circuit board between Vout and vload. Therefore, although there are changes in line, load and temperature, the ltm4601 can still ensure that the voltage accuracy of the load is ± 2.0% or higher.
But please note that remote detection is not a panacea. In fact, it places a large feedback loop between the power supply and the load. If you think of the power regulator as a power operational amplifier, this feedback loop will expose the power supply to noise and EMI / RFI, affecting the closed-loop performance. Even, the existence of this loop may lead to instability and oscillation of the regulator. Therefore, the layout must be carefully considered when implementing remote detection.
Another way to minimize the impact of IR is to use multiple smaller regulators close to each load, rather than concentrating a larger regulator in one location. This leads to a classic trade-off about tangible "cost", that is, whether to use two or more smaller and cheaper units or one larger and more expensive unit. Although the BOM cost difference can be quantified, the technical impact of selecting a large device and multiple small devices is difficult to evaluate, and needs analysis, judgment and experience.
For example, the lmzm33602 power module of Texas Instruments uses a relatively small flat package, which combines the step-down converter with power MOSFET, shielded inductor and passive components to provide 1 to 18 volts and up to 2 amps (Figure 1). It only needs four to five non critical external passive components, and eliminates the loop compensation and magnetic problems in the regulator design.
The lmzm33602 is packaged in QFN and the size is only 9 mm × 7 mm × 4 mm, which can be easily attached near the load element or sub circuit. This minimizes the IR pressure drop in two ways.
First, it is close to the load, which can reduce the power rail resistance and noise pickup. Secondly, the output current is only a few amps, which also reduces the IR voltage drop. Therefore, deploying multiple such units can provide greater layout flexibility, reduce IR voltage drop, reduce noise pickup, improve distributed cooling, and other system level advantages than using a single larger 10 Amp unit.
Isolation: sometimes optional, but usually mandatory
The need for current isolation (i.e. no resistance path between the two parts of the circuit) may be advantageous or mandatory to some extent. Eliminating the system ground loop may be helpful because it may be necessary to connect a "floating" (ungrounded) transmitter, or it must be done to ensure safety between the high voltage circuit and the user of the medical device. For many designers, the necessity or function of this isolation is either unclear or difficult to understand.
Regardless of the basic principle, it is often overlooked that the isolation sub circuit also needs to isolate the power supply, which usually maintains a relatively low current level. In the past, even the demand for low current isolated power supply required a lot of circuit board space, and its BOM cost was usually too high compared with other functions. Choosing the "self built" approach rather than "purchase" is usually not feasible because isolation design has a great impact on design or assembly. In addition, for many applications, isolation design and physical implementation need to be tested and certified to meet industry and regulatory standards. This is an expensive and complex process.
However, due to the emergence of small and fully compliant and certified isolated DC / DC modules (such as ltm8047 of analog devices, figure 3), designers can overcome this problem to a great extent. Using isolated flyback topology, 725 VDC isolation can be provided.
At its size of 11.25 mm × 9 mm × The 4.92 mm micro BGA package encapsulates the switch controller, power switch and all supporting components, as well as the core components of the isolation transformer (Fig. 4). It provides a 2.5 to 12 volt output (always in Buck mode) over a wide input voltage range of 3.1 to 32 volts. Although it can only provide medium current, 440 Ma at 2.5 VDC, this is enough to power many isolated sub circuits and transmitter front ends.
Adjustability: useful, but be careful
These off the shelf DC / DC Regulators rarely provide a fixed preset voltage. The user can set the voltage using a voltage divider consisting of a pair of resistors. This has many advantages: the same regulator can be used in many locations, thus simplifying the BOM; The output voltage can be "raised" by a few MV to compensate for the IR voltage drop (in many cases, this is not recommended, but it is very common); And the output voltage can be adjusted upward to adapt to the required settings in analog circuits, especially RF, which involves specifying the trade-off between performance and power consumption (high voltage provides better SNR and wider bandwidth, but at the cost of increasing power consumption).
However, the user needs to recognize that the stability and temperature coefficient of the voltage setting resistor and the thermal operating environment must be taken into account in any calculation of the nominal DC output voltage of the regulator. At higher temperatures, the DC power rail may drift out of the load specification range. Therefore, it may be wise or necessary to select a voltage setting resistor with low temperature coefficient for this function, rather than using a general-purpose device such as pull-up resistor, which may only be suitable for other non critical functions.
Another setting function provided by some DC / DC regulators is to select the switching frequency (all these regulators use the switching topology; For reasons of efficiency and size, they are not low dropout regulators (i.e. LDOS). For example, the max17536 of Maxim integrated can be set by a single resistor and operates in a wide frequency range of 100 kHz to 2.2 MHz (Figure 5). In this way, it can be set to avoid the impact of its switching noise on nearby circuits with frequency overlap, such as AM radio band of 550 to 1600 kHz, or avoiding opening a specific narrowband containing related signals.
Note that the relationship between resistance and switching frequency is nonlinear and not too accurate. For this and other reasons, the max17536 can be set to synchronize with the external power supply instead of operating at the frequency set by its resistance. This also avoids accidental frequency mixing with other clock sources in the system and the resulting beat frequency, which may lead to subtle and difficult to diagnose problems.
summary
These complete, compact DC / DC converters eliminate most of the risks and challenges of designing low-voltage, medium current (1 amp or less to 10 amps) power supplies. However, like any component, there are some recognized basic rules, and usually only by following these rules can we achieve successful installation, realize its full potential and avoid the "trap"“
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